Some aspects of electronic topological transition in 2D system on a square lattice. Excitonic ordered states

We study the ordered "excitonic" states which develop around the quantum critical point (QCP) associated with the electronic topological transition (ETT) in a 2D electron system on a square lattice. We consider the case of hopping beyond nearest neighbors when ETT has an unusual character. We show that the amplitude of the order parameter (OP) and of the gap in the electron spectrum increase with increasing the distance from the QCP, \delta_c - \delta, where \delta = 1-n and "n" is an electron concentration. Such a behavior is different from the ordinary case when OP and the gap decrease when going away from the point which is a motor for instability. The gap opens at "hot spots" and extends untill the saddle points (SP) whatever is the doping concentration. The spectrum gets a characteristic flat shape as a result of hybrydization effect in the vicinity of two different SP's. The shape of the spectrum and the angle dependence of the gap have a striking similarity with the features observed in the normal state of the underdoped high-T$_c$ cuprates. We discuss also details about the phase diagram and the behaviour of the density of states.Comment: 15 pages, 14 EPS figures included, EPJ style included, added references, changed conten

Chiral Magnetic Skyrmions with Arbitrary Topological Charge ("skyrmionic sacks")

We show that continuous and spin-lattice models of chiral ferro- and antiferromagnets provide the existence of an infinite number of stable soliton solutions of any integer topological charge. A detailed description of the morphology of new skyrmions and the corresponding energy dependencies are provided. The considered model is general, and is expected to predict a plethora of particle-like states which may occur in various chiral magnets including atomic layers, e.g., PdFe/Ir(111), rhombohedral GaV$_4$S$_8$ semiconductor, B20-type alloys as Mn$_{1-x}$Fe$_x$Ge, Mn$_{1-x}$Fe$_x$Si, Fe$_{1-x}$Co$_x$Si, Cu$_2$OSeO$_3$, acentric tetragonal Heusler compounds

Various ordered states in a 2D interacting electron system close to an electronic topological transition

We consider a 2D electron system on a square lattice with hopping beyond nearest neighbors. The existence of the quantum critical point associated with an electronic topological transition in the noninteracting system results in density wave (DW) and high temperature d-wave superconducting (dSC) instabilities in the presence of an exchange interaction J. We analyse different DW ordering such as isotropic Spin DW (SDW), d-wave SDW, isotropic Charge DW (CDW) and d-wave CDW. The coexistence of dSC and SDW orders leads necessary to the existence of a third order which is a pi triplet superconducting (PTS) order. A new phase diagram with a mixed phase of SDW, dSC and PTS order is found. The theory is applied to high-Tc cuprates.Comment: 2 pages, 1 figure, submitted to LT22 (Physica B

Multipolar Origin of the Unexpected Transverse Force Resulting from Two-Wave Interference

We propose a theoretical study on the electromagnetic forces resulting from the superposition of a TE and TM plane waves interacting with a sphere. Specifically, we first show that, under such an illumination condition, the sphere is subjected to a force transverse to the propagation direction of the waves. We then analyze the physical origin of this counter-intuitive behavior using a multipolar decomposition of the electromagnetic modes involved in that scattering process. This analysis reveals that interference effects, due to the two-wave illumination, lead to a Kerker-like asymmetric scattering behavior resulting in this peculiar transverse force

Leptonic constant of pseudoscalar B_c meson

We calculate the leptonic constant for the ground pseudoscalar state of B_c meson in the framework of QCD-motivated potential model taking into account the two-loop anomalous dimension for the heavy quark current in the nonrelativistic QCD as matched with the full QCD.Comment: 6 pages, RevTeX4 file, 3 eps-figures, several references and remarks adde

Effective action for the Kondo lattice model. New approach for S=1/2

In the partition function of the Kondo lattice, spin matrices are exactly replaced by bilinear combinations of Fermi operators with the purely imaginary chemical potential lambda=-i.pi.T/2 (Popov representation). This new representation of spin operators allows one to introduce new Green's functions with Matsubara frequencies 2.pi.T(n+1/4) for S=1/2. A simple temperature diagram technique is constructed with the path integral method. This technique is standard and does not contain the complicated combinatoric rules characteristic of most of the known variants of the diagram techniques for spin systems. The effective action for the almost antiferromagnetic Kondo lattice is derived.Comment: 7 pages, Proceedings of SCES98/Paris; one reference adde

New type of stable particle like states in chiral magnets (Chiral bobbers)

We present a new type of a thermodynamically stable magnetic state at interfaces and surfaces of chiral magnets. The state is a soliton solution of micromagnetic equations localized in all three dimensions near a boundary and contains a singularity, but nevertheless has a finite energy. Both features combine to a quasi-particle state for which we expect unusual transport and dynamical properties. It exhibits high thermal stability and thereby can be considered as promising object for fundamental research and practical applications in spintronic devices. We provide arguments that such a state can be found in different B20-type alloys e.g. Mn$_{1-x}$Fe$_x$Ge, Mn$_{1-x}$Fe$_x$Si, Fe$_{1-x}$Co$_x$Si.Comment: accepted in PR

Asymmetric spin-1/2 two-leg ladders

We consider asymmetric spin-1/2 two-leg ladders with non-equal antiferromagnetic (AF) couplings J_|| and \kappa J_|| along legs (\kappa <= 1) and ferromagnetic rung coupling, J_\perp. This model is characterized by a gap \Delta in the spectrum of spin excitations. We show that in the large J_\perp limit this gap is equivalent to the Haldane gap for the AF spin-1 chain, irrespective of the asymmetry of the ladder. The behavior of the gap at small rung coupling falls in two different universality classes. The first class, which is best understood from the case of the conventional symmetric ladder at \kappa=1, admits a linear scaling for the spin gap \Delta ~ J_\perp. The second class appears for a strong asymmetry of the coupling along legs, \kappa J_|| << J_\perp << J_|| and is characterized by two energy scales: the exponentially small spin gap \Delta ~ J_\perp \exp(-J_|| / J_\perp), and the bandwidth of the low-lying excitations induced by a Suhl-Nakamura indirect exchange ~ J_\perp^2 /J_|| . We report numerical results obtained by exact diagonalization, density matrix renormalization group and quantum Monte Carlo simulations for the spin gap and various spin correlation functions. Our data indicate that the behavior of the string order parameter, characterizing the hidden AF order in Haldane phase, is different in the limiting cases of weak and strong asymmetry. On the basis of the numerical data, we propose a low-energy theory of effective spin-1 variables, pertaining to large blocks on a decimated lattice.Comment: 18 pages, 11 figure

Mass spectra of doubly heavy Omega_QQ' baryons

We evaluate the masses of baryons composed of two heavy quarks and a strange quark with account for spin-dependent splittings in the framework of potential model with the KKO potential motivated by QCD with a three-loop beta-function for the effective charge consistent with both the perturbative limit at short distances and linear confinement term at long distances between the quarks. The factorization of dynamics is supposed and explored in the nonrelativistic Schroedinger equation for the motion in the system of two heavy quarks constituting the doubly heavy diquark and the strange quark interaction with the diquark. The limits of approach, its justification and uncertainties are discussed. Excited quasistable states are classified by the quantum numbers of heavy diquark composed by the heavy quarks of the same flavor.Comment: 14 pages, revtex4-file, 3 eps-figures, 5 tables, typos correcte