Duality and quantum-algebra symmetry of the AN−1 open spin chain with diagonal boundary fields

We show that the transfer matrix of the A_N−1⁽¹⁾ open spin chair with diagonal boundary fields has the symmetry U_q(SU(l)) × U_q(SU(N−l)) × U(1), as well as a “duality” symmetry which maps l ↔ N − l. We exploit these symmetries to compute exact boundary S-matrices in the regime with q real.     

Scattering-controlled recombination of Δ2-light-hole indirect excitons and apparently enhanced quantum confined Stark effect in tensilely strained Si1−yCy/Si (0 0 1) quantum wells

Anomalies are found in the near-band-edge luminescence properties of Δ₂-light-hole indirect excitons in Si_1−yC_y-based tensilely strained quantum wells (QWs). The experimental spectra exhibit a clear signature of phonon-assisted transitions on the lower energy side of the “no-phonon” transition, which indicates the relevance of “virtual” indirect valleys and in-plane k-dispersion, as opposed to the theoretical prediction that the zone-centered Δ₂ valleys take over the conduction band edge. Intervalley scattering between [0 0 1]-Δ₂ valleys and in-plane Δ₄ valleys is suggested as the underlying mechanism. On the other hand, the experimental evidence was found for “apparently enhanced” quantum-confined Stark red shifts for Si_1−yC_y-based QWs. However, quantitative estimates are in conflict with the experimental results and predict a blue shift due to exciton weakening which masks the Stark effect as in the case of Δ₄-heavy hole excitons in Si_1−xGe_x-based QWs.     

Neutrino oscillations induced by two-loop radiative mechanism

Two-loop radiative mechanism, when combined with an U(1)_L′ symmetry generated by L_e − L_μ − L_τ (=L′), is shown to provide an estimate of Δm²/Δm²_atm εm_e/m_τ, where ε measures the U(1)_L′-breaking. Since Δm²_atm 3.5×10⁻³ eV², we find that Δm² ε10⁻⁶ eV², which will fall into the allowed region of the LOW solution to the solar neutrino problem for ε 0.1.     

A Pati–Salam model from branes

We explore the possibility of embedding the Pati–Salam model in the context of Type I brane models. We study a generic model with U(4)_C×U(2)_L×U(2)_R gauge symmetry and matter fields compatible with a Type I brane configuration. Examining the anomaly cancellation conditions of the surplus abelian symmetries we find an alternative hypercharge embedding that is compatible with a low string/brane scale of the order of 5–7 TeV, when the U(4)_C and U(2)_R brane stack couplings are equal. Proton stability is assured as baryon number is associated to a global symmetry remnant of the broken abelian factors. It is also shown that this scenario can accommodate an extra low energy abelian symmetry that can be associated to lepton number. The issue of fermion and especially neutrino masses is also discussed.     

The pion velocity at chiral restoration and the vector manifestation

We study the effects of Lorentz non-invariance on the physical pion velocity at the critical temperature T_c in an effective theory of hidden local symmetry (HLS) with the “vector manifestation” fixed point. We match at a “matching scale” Λ_M the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the physical pion velocity, which is found to be one at T=T_c when starting from the Lorentz invariant bare HLS, remains close to one with the Lorentz non-invariance, v_π(T_c)=0.83–0.99. This result is quite similar to the pion velocity in dense matter.     

Magnetic anisotropy and superconductivity in a model for high-Tc copper oxides

The phase diagram for the CuO₂-based superconductors is found to be consistent with an extended Hubbard Hamiltonian with competing positive-and negative-U interactions on a 2D lattice where sites are plaquettes formed by clusters of Cu and O atoms. The negative-U effective interactions are implied by the XY anisotropy in the Cu-Cu spin couplings and local hole pairing corresponds to vortex-antivortex spin configurations. The phase progression observed with the variation of dopant fraction x can be obtained via gradual implementation of canonical transformation that maps the properties of the positive-U Hubbard model at half-filling into those of the negative-U model away from half-filling. In the strong-coupling limit this process is described in terms of percolation-driven dilute magnetism for both spins (U>0) and pseudospins (U−1_x=ξ⁻¹_o+η⁻¹_x for x→O as seen in La_2-xSr_xCuO₄. (ii) An x-dependent reduction of spin fluctuations at low temperatures that conforms with NMR studies of La_2-xSr_xCuO₄. And, (iii) a reduced superconducting transition locus T_c(x)/T_cmax in agreement with the universal shape and location revealed by analysis of experimental data.     

Interrelation of superconducting and antiferromagnetic gaps in high- T(c) compounds: A test case for the SO(5) theory

Recent angle resolved photoemission data, which found evidence for a d-wave-like modulation of the antiferromagnetic gap, suggest an intimate interrelation between the antiferromagnetic insulator and the superconductor with its d-wave gap. It is shown here that a projected SO(5) theory, which explicitly takes the Mott-Hubbard gap into account, correctly describes the observed gap characteristics. Specifically, it accounts for the order of magnitude difference between the antiferromagnetic gap modulation and the superconducting gap and is also consistent with the gap dispersion.     

Thermal fluctuations in d-wave layered superconductors

We study the thermal fluctuations of anisotropic order parameters (OP) in layered superconductors. In particular, for copper oxides and a d-wave OP, we present some experimental consequences of fluctuations in the direction normal to the layers. It is shown that the c-axis penetration depth λ_c can have a “disorder-like” quadratic temperature dependence at low temperature. The fluctuations are analyzed in the framework of a Lawrence-Doniach model with an isotropic Fermi surface. Anisotropies pin the orientation of the OP to the crystallographic axes of the lattice. Then we study an extended t-J model that fits Fermi surface data of bilayers YBCO and BSCCO. This leads to a d-wave OP with two possible orientations and, including the thermal fluctuations, yields the announced temperature dependence of λ_c. Furthermore a reservoir layer is introduced. It implies a finite density of states at the Fermi energy which is successfully compared to conductance and specific heat measurements.     

Matter–antimatter asymmetry and neutrino properties

The cosmological baryon asymmetry can be explained as remnant of heavy Majorana neutrino decays in the early universe. We study this mechanism for two models of neutrino masses with a large ν_μ−ν_τ mixing angle which are based on the symmetries SU(5)×U(1)_F and SU(3)_c×SU(3)_L×SU(3)_R×U(1)_F, respectively. In both cases B−L is broken at the unification scale Λ_GUT. The models make different predictions for the baryogenesis temperature and the gravitino abundance.     

Projective and affine connections on S and integrable systems

It is known that the Korteweg–de Vries (KdV) equation is a geodesic flow of an L² metric on the Bott–Virasoro group. This can also be interpreted as a flow on the space of projective connections on S¹. The space of differential operators Δ⁽ⁿ⁾=∂ⁿ+u₂∂ⁿ⁻²++u_n form the space of extended or generalized projective connections. If a projective connection is factorizable Δ⁽ⁿ⁾=(∂−((n+1)/2−1)p₁)(∂+(n−1)/2p_n) with respect to quasi primary fields p_i’s, then these fields satisfy ∑_i=1ⁿ((n+1)/2−i)p_i=0. In this paper we discuss the factorization of projective connection in terms of affine connections. It is shown that the Burgers equation and derivative non-linear Schrödinger (DNLS) equation or the Kaup–Newell equation is the Euler–Arnold flow on the space of affine connections.     

Magnetic order driven by orbital ordering in the semiconducting KFe1.5Se2

The two-orbital Hubbard model is studied numerically by using the Hartree-Fock approximation in both real space and momentum space, and the ground-state properties of the alkali metal iron selenide semiconducting KFe_1.5Se₂ are investigated. A rhombus-type Fe vacancy order with stripetype antiferromagnetic (AFM) order is found, as was observed in neutron scattering experiments [J. Zhao, et al., Phys. Rev. Lett. 109, 267003 (2012)]. Hopping parameters are obtained by fitting the experimentally observed stripe AFM phase in real space. These hopping parameters are then used to study the ground-state properties of the semiconductor in momentum space. It is found to be a strongly correlated system with a large on-site Coulomb repulsion U, similar to the AFM Mott insulator — the parent compound of copper oxide superconductors. We also find that the electronic occupation numbers and magnetizations in the d_xz and d_yz orbitals become different simultaneously when U>U_c (～3.4 eV), indicating orbital ordering. These results imply that the rotational symmetry between the two orbitals is broken by orbital ordering and thus drives the strong anisotropy of the magnetic coupling that has been observed by experiments and that the stripe-type AFM order in this compound may be caused by orbital ordering together with the observed large anisotropy.     

Tunneling studies on NdBa2Cu3O7−y

Tunneling measurements were performed at 4.2 K on well-characterized NdBa₂Cu₃O_7−y samples using a sandwich configuration with an artificially grown barrier layer (sputtered indium oxide) and Pb_0.5In_0.5 counter electrode. The conductance spectra exhibited well-defined structures characteristic of gap opening. Fitting the data to a life-time broadened BCS density of states function yielded the following values: 20 meV for the energy gap Δ, 1.5 meV for the line width Γ and 5.3 for 2Δ/k_BT_c.     

A reduced hamiltonian model for U(∞) gauge theories

A reduced version of the Kogut-Susskind gauge theory is presented for U(∞) gauge theories. The reduced hamiltonian , where V_ij=trU_iD_iD_jD_j(U_iD_j) ⁺ (U_iD_j)⁺, is obtained by representing the group of space translations in the diagonal part of U(∞) U(∞). The space of states is invariant to the reduced gauge transformation U_i→ωU_iD_iω⁺D_i⁺ and the scalar product carries a gauge invariant constraint on U_i The hamiltonian is also expr essed in terms of the string variables of Bars. We present the linear potential between static quarks at strong coupling and sketch the weak coupling expansion.     

Oxygen nonstoichiometry and phase transitions of the neodymium-rich Nd1+xBa2−xCu3Oz solid solution

On the basis of chemical, thermal analysis and Cu K-edge X-ray absorption measurements, oxygen content in the Nd_1+xBa_2−xCu₃O_z solid solution was determined between 1000°C in air and 400°C in oxygen for x=0.05–0.9 compositions. It has been observed that the oxygen nonstoichiometry Δz of the Nd_1+xBa_2−xCu₃O_7+x/2−Δz solid solution decreases 2–2.5 times for a large substitution (Δz≈0.3–0.33 for x=0.9), despite of the acclaimed higher total oxygen content. The difference in nonstoichiometry is explained by a higher average value of the copper oxidation state (ACV), which is vital for the solid solution with large x even at elevated temperatures (ACV≈2–2.05 for x>0.3 at 1000°C, PO₂=0.21 atm). On the contrary, the ACV after complete oxygenation is almost constant (about 2.25–2.3) for the whole series. The x-dependence of the oxygen content is not monotonous and structural phase transitions can be observed at x=0.3 and x=0.6, as confirmed by the X-ray diffraction and the Raman scattering spectroscopy. The first well-known transition is connected with the oxygen disorder due to the Nd substitution for Ba at random Ba-sites. In the present work, it is proved by the apical oxygen mode broadening in Raman spectra. Ordering of the Nd and Ba atoms with a subsequent orthorhombic distortion of the lattice may occur even at 1000°C in air due to the second transformation at x≈0.6. The invariable orthorhombicity of the Nd-rich solid solution with x>0.6 is not caused by the oxygen absorption as in the x=0.05 case. Existence of high- and low-temperature orthorhombic modifications of this solid solution has been observed for the first time. Finally, a tentative 3D (z–x–T) diagram is suggested for the Nd_1+xBa_2−xCu₃O_z solid solution up to 1000°C in air, including the new x=0.6–0.9 region.     

Gauge-Higgs unification in the left–right model

We construct a supersymmetric left–right model in four dimension with gauge-Higgs unification starting from a SU(3)_c×SU(4)_w×U(1)_B−L gauge symmetry in five dimension. The model has several interesting features, such as, the CKM mixings in the quark sector are naturally small while for the neutrino sector it is not, light neutrino masses can be generated via the seesaw mechanism in the usual way, and the model has a U(1)_R symmetry which naturally forbid dimension five proton decay operators. We also discuss the grand unification of our model in SO(12) in five dimensions.     

Universal enveloping algebra and differential calculi on inhomogeneous orthogonal q-groups

We review the construction of the multiparametric quantum group ISO_q,r(N) as a projection from SO_q,r (N + 2) and show that it is a bicovariant bimodule over SO_q,r(N). The universal enveloping algebra U_q,r(iso(N)), characterized as the Hopf algebra of regular functionals on ISO_q,r(N), is found as a Hopf subalgebra of U_q,r(so(N + 2)) and is shown to be a bicovariant bimodule over U_q,r(so(N)).

An R-matrix formulation of U_q,r(iso(N)) is given and we prove the pairing U_q,r(iso(N)) — ISO_q,r(N)). We analyze the subspaces of U_q,r(iso(N)) that define bicovariant differential calculi on ISO_q,r(N).     

Molecular associations in binary mixtures of pyridine and chlorobenzene in benzene solution using microwave absorption data

The dielectric relaxation time (τ) of binary mixtures of different molar concentrations of pyridine (C₅H₅N) and chlorobenzene (C₆H₅Cl) in benzene solution at different temperatures (25, 30, 35 and 40 °C) has been calculated by using standard microwave techniques and Gopala Krishna's single frequency (9.875 GHz) concentration variation method. The energy parameters (ΔH_ε, ΔF_ε, and ΔS_ε) for the dielectric relaxation process of the binary mixture containing 0.5 mol fraction of pyridine have been calculated at the respective temperatures. Comparisons have been made with the corresponding energy parameters for the viscous flow (ΔH_η, ΔF_η, and ΔS_η). From the observations it is found that the dielectric relaxation process can be treated as the rate process. Based upon above studies, solute–solvent type of molecular associations arising from the interaction of chlorobenzene and benzene and pyridine and benzene molecules has been proposed. No solute–solute type of molecular association has been observed.     

Charge susceptibility and charge correlation function in the d---p model

The properties of the charge fluctuation are investigated in the d---p model with the repulsion U_pd between holes on the nearest-neighbor Cu and O sites and the infinite on-site repulsion U_d at the Cu site. We calculate the charge susceptibility χ^c(q, iω_n) and the charge correlation function S^c(q) = TΣ_ωn χ^c(q, iω_n). It is found that S^c(q) has a peak at the Γ point and a maximum in a ring around the Γ point. The former is due to Tχ^c(q, 0). Its intensity is proportional to temperature T and strongly enhanced by U_pd. The latter is due to TΣ_{ωn ≠ 0} χ^c(q, iω_n) and shows a weak T and U_pd dependence. The intensity of the diffuse X-ray scattering on taking the charge fluctuation into account is also calculated. The result is consistent with the experiments in La_2−δSr_δCuO₄.     

Z′ mediated flavor changing neutral currents in B meson decays

We study the effects of an extra U(1)′ gauge boson with flavor changing couplings with fermion mass eigenstates on certain B meson decays that are sensitive to such new physics contributions. In particular, we examine to what extent the current data on B_d→φK and B_d→η′K decays may be explained in such models, concentrating on the example in which the flavor changing couplings are left-chiral. We find that within reasonable ranges of parameters, the Z′ contribution can readily account for the anomaly in S_φKS but is not sufficient to explain large branching ratio of B_d→η′K with the same parameter value. S_φKS and S_η′KS are seen to be the dominant observables that constrain the extra weak phase in the model.     

Yb (Ba, Sr)2Cu4O8: a “124” phase superconductor with a reduced unit cell volume

Yb(Ba_1−xSr_x)₂Cu₄O₈ (0.1x0.3) superconductors of the YBa₂Cu₄O₈(“124”) structure were successfully synthesized using an O₂-hot-isostaticc-pressing (HIP) technique. The samples were characterized with respect to the crystal structure and superconducting properties. The lattice parameters of the samples decreased as the substitution of Sr for Ba proceeded. The superconducting transition temperatures of all the Yb-“124” samples were more or less the same, being around 80 K.