Meson screening masses from lattice QCD with two light quarks and one strange quark

We present results for screening masses of mesons built from light and strange quarks in the temperature range of approximately between 140 MeV to 800 MeV. The lattice computations were performed with 2+1 dynamical light and strange flavors of improved (p4) staggered fermions along a line of constant physics defined by a pion mass of about 220 MeV and a kaon mass of 500 MeV. The lattices had temporal extents N _τ =4, 6 and 8 and aspect ratios of N _s /N _τ ≥4. At least up to a temperature of 140 MeV the pseudo-scalar screening mass remains almost equal to the corresponding zero temperature pseudo-scalar (pole) mass. At temperatures around 3T _c (T _c being the transition temperature) the continuum extrapolated pseudo-scalar screening mass approaches very close to the free continuum result of 2πT from below. On the other hand, at high temperatures the vector screening mass turns out to be larger than the free continuum value of 2πT. The pseudo-scalar and the vector screening masses do not become degenerate even for a temperature as high as 4T _c . Using these mesonic spatial correlation functions we have also investigated the restoration of chiral symmetry and the effective restoration of the axial symmetry. We have found that the vector and the axial-vector screening correlators become degenerate, indicating chiral symmetry restoration, at a temperature which is consistent with the QCD transition temperature obtained in previous studies. On the other hand, the pseudo-scalar and the scalar screening correlators become degenerate only at temperatures larger than 1.3T _c , indicating that the effective restoration of the axial symmetry takes place at a temperature larger than the QCD transition temperature.     

Gauge fixing, zero-momentum modes and the calculation of masses on a lattice

It is shown that the zero-momentum modes can strongly affect the values of the masses, for example the magnetic screening mass m_m, calculated from gauge-dependent correlators with zero momentum.     

Bootstrap calculation of the dynamical quark mass in QCD4 at finite temperature

Nonperturbative calculations of the dynamical quark mass m(T) are given in QCD₄, based on the bootstrap solution of the Schwinger-Dyson equation for the quark Green function at finite temperatures. A closed nonlinear equation is obtained for m(T) whose solution is found under some simplifying assumptions. We used a particular approximation for the effective charge and the nonperturbative expressions of the gluon magnetic and electric masses. The singular behavior of m(T) is established and its parameters are determined numerically. The singularity found is shown to correctly reproduce the chiral phase transition and the temperature limits obtained for m(T) are qualitatively correct. The complete phase diagram of QCD₄ in the (μ, T) plane is briefly discussed.     

New isomers and their decay in odd-odd neutron-deficient cesium isotopes

By a systematic on-line cesium mass separation fromA=122 toA=132 and subsequent gamma and electron decay spectroscopy at very low energy, new isomers have been precisely identified:^122m Cs (T _1/2=0.36±0.02 s),^124m Cs (T _1/2=6.3±0.2 s) and^130m Cs (T _1/2=3.46±0.06 min). Detailed level schemes are given for both^124m Cs and^130m Cs. Comparison of excited levels known in the odd-odd nuclei of the same region shows that more experimental informations are needed to propose a clear and realistic picture of the nuclear states structure. Radioactivity:^{122m, 124m, 130m} Cs [from La or Ce,³Hexn]-measuredT _1/2,E _γ,I _γ,E _CE ,I _CE ,γ-γ-t, γ-ce coinc.; deduced ICC,^{124, 130}Cs deduced levels,J, π. Online mass separated sources, Ge(Li), intrinsic Ge, Si(Li), magnetic electron selector.     

A study of 't Hooft and Wilson loops

An investigation is undertaken for 't Hooft loop operators in four-dimensional gauge theories. For the first time, a perimeter law is shown to be their behavior in weak coupling Wilson lattice (and continuum) non-abelian SU(N) gauge theories for all N. However, it is also argued that this perimeter law is poor criterion for quark confinement. Rather, it is suggested that non-leading long-distance behavior is what is crucial and relevant in distinguishing non-abelian from abelian (and hence confining from non-confining) theories. A new object, “the 't Hooft line”, is introduced to measure this non-leading behavior and is computed in strong coupling on the lattice. There, one finds magnetic screening characterized by a magnetic screening mass, m_s. It is shown to all orders in strong coupling that m_s is the glueball mass, a result which is expected to persist in weak coupling and in the continuum. Two further consequences of this work are that pure non-abelian gauge theories cannot be in a Higgs phase and that in such models that absence of massless physical particles implies confinement.Finally, non-leading behavior in Wilson loops is examined. The present picture of confinement suggests the absence of van der Waals forces in Yang-Mills theories.     

Two-dimensional Heisenberg model with spin s=1/2 and antiferromagnetic exchange treated as a spin liquid

The spin system of the Heisenberg model (s=1/2) on a square lattice with antiferromagnetic (AFM) exchange between nearest neighbors (in which there is no long-range magnetic order at any T≠0) is treated as a spatially homogeneous isotropic spin liquid. The double-time temperature Green’s function method is used in the framework of a second-step decoupling scheme. It is shown that, as T → 0, the spin liquid goes over (without any change in symmetry) to a singlet state with energy (per bond) ?₀=?0.352 and the correlation length diverges as ξ ∝ T ^?1 exp(T ₀/T). The spatial spin correlators oscillate in sign with distance, as in the AFM state. The theory allows one to calculate the main characteristics of the system in all temperature ranges.     

Emission Mössbauer studies of the magnetoresistive compound, La0.7Sr0.3MnO3

The magnetic behavior of the Sr_0.3 manganite is studied using a local microprobe, ⁵⁷Co. In contrast with Ca substituted manganites, a much larger fraction of the material exhibits short-range order with superparamagnetic-like behavior even at 80 K. The differences in behavior are attributed to the large mismatch between the ionic radii of La⁺³ and the divalent substituent Sr⁺², which introduces anharmonicity in local vibrations. In common with all other compounds exhibiting negative bulk magnetoresistivity, the Sr_0.3 compound also exhibits very marked softening of lattice as one approaches T_c from below. Application of an external magnetic field results in coalescing of nanosized magnetic clusters to form larger ones with better alignment of spins.     

High-field phase-diagram of Fe arsenide superconductors

Here, we report an overview of the phase-diagram of single-layered and double-layered Fe arsenide superconductors at high magnetic fields. Our systematic magneto-transport measurements of polycrystalline SmFeAsO_1-xF_x at different doping levels confirm the upward curvature of the upper critical magnetic field H_c2(T) as a function of temperature T defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single-crystals that this feature, which was interpreted in terms of the existence of two superconducting gaps, is ubiquitous among both series of single- and double-layered compounds. In all compounds explored by us the zero temperature upper critical field H_c2(0), estimated either through the Ginzburg–Landau or the Werthamer–Helfand–Hohenberg single gap theories, strongly surpasses the weak-coupling Pauli paramagnetic limiting field. This clearly indicates the strong-coupling nature of the superconducting state and the importance of magnetic correlations for these materials. Our measurements indicate that the superconducting anisotropy, as estimated through the ratio of the effective masses γ =  (m_c/m_ab)^1/2 for carriers moving along the c-axis and the ab-planes, respectively, is relatively modest as compared to the high-T_c cuprates, but it is temperature, field and even doping dependent. Finally, our preliminary estimations of the irreversibility field H_m(T), separating the vortex-solid from the vortex-liquid phase in the single-layered compounds, indicates that it is well described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor.     

I=1/2 low-lying mesons in lattice QCD

Using a conventional constituent-quark model,I=1/2 scalarκ,vector K*(892),and axial vector K1mesons are studied in the asqtad-improved staggered fermion with wall-source and point-sink interpolators.The mass ratio of mκ/mK*(892)is numerically confirmed to apparently vary with quark mass,and the experimental ordering mK*(892)mκholds elegantly when the light u/d quark masses are sufficiently small,while the valence strange quark mass is fixed to its physical value.We also get reasonable signals for the K1 meson suggested by the SCALAR Collaboration from lattice QCD.The computations are conducted with the MILC Nf=3 flavor gauge configurations at three lattice spacings:a≈0.15,0.12,and 0.09 fm.     

Galvanomagnetic properties of atomically disordered Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript> single crystals

The effect of neutron-bombardment-induced atomic disorder on the galvanomagnetic properties of Sr₂RuO₄ single crystals has been experimentally studied in a broad range of temperatures (1.7–380 K) and magnetic fields (up to 13.6 T). The disorder leads to the appearance of negative temperature coefficients for both the in-plane electric resistivity (ρ_a) and that along the c axis (ρ_c), as well as the negative magnetoresistance Δρ, which is strongly anisotropic to the magnetic field orientation (H ∥ a and H ∥ c), with the easy magnetization direction along the c axis and a weak dependence on the probing current direction in the low-temperature region. The experimental ρ_a(T) and ρ_c(T) curves obtained for the initial and radiation-disordered samples can be described within the framework of a theoretical model with two conductivity channels. The first channel corresponds to the charge carriers with increased effective masses (～10m _e , where m _e is the electron mass) and predominantly electron-electron scattering, which leads to the quadratic temperature dependences of ρ_a and ρ_c. The second channel corresponds to the charge carriers with lower effective masses exhibiting magnetic scattering at low temperatures, which leads to the temperature dependence of the ρ_{a, c}(T) ∝ 1/T type.     

Possible mechanism of superconductivity in the CuO-Cu interface

Apparently, a two-dimensional CuO lattice is formed on the surface of copper oxide in the CuO-Cu interface. This lattice consists of Cu²⁺ and O^1? ions, which form a narrow, partially filled two-dimensional band. In this case, local electron pairs (LEPs) can form in the oxygen subsystem as a result of the fulfillment of the Shubin-Vonsovskii conditions. A crude estimate of the formation temperature of LEPs gives T* ～ 10 4 K. At the concentration in the interface layer n～1.6×10²⁰ cm^?3 and the effective mass of carriers m* ～ m _e, the onset temperature of Bose-Einstein condensation may take a value of T _c ～ 1000 K. The estimate obtained for the temperature T _c corresponds to the experimental value by an order of magnitude.     

Nuclear magnetic resonance on oriented 11/2− 129mXe and131mXe in Fe

The magnetic hyperfine splitting frequenciesν _M=¦gμ _NB_HF/h.¦ of the 11/2^? isomeric states^129m Xe (T_1/2=8.9d) and^131mXe (T_1/2=11.8d) in Fe were measured with nuclear magnetic resonance on oriented nuclei at temperatures of 10–15 mK as 188.0(1) MHz and 209.8(1) MHz, respectively, the samples being prepared with the technique of recoil implantation after (α, x n) reactions. The magnetic moments of^129m Xe and^131m Xe are deduced to be (?)0.8914(6)μ _N and (?)0.9943(6)μ _N, respectively. The missing γ-anisotropies for allγ-transitions following the decay of 36.4d ¹²⁷Xe indicateI=1/2 for the ground state spin of¹²⁷Xe.     

Spin crossover in the magnetic phase transition in YBa2(Cu1−x Fex)3O7±δ superconductors

All iron ions in the Cu1 and Cu2 local lattice sites of the YBa₂(Cu_0.9 ⁵⁷Fe_0.1)₃O_7.01 superconductor with T _c=31 K experienced magnetic ordering below T _m=22 K. Therefore, at T < T _m, magnetic ordering coexisted with superconductivity. According to the Mössbauer spectroscopy data, iron ions in Cu2 (Fe2) sites were in the low-spin state at T < T _m(S= 3/2 or 1/2), whereas an equal number of iron ions in Cu1 (Fe1) sites were in the high-spin Fe³⁺ state (S=5/2). The magnetic transition near T _m changed iron ion spin states-low-spin ions turned into high-spin ions, and vice versa. This preserved the spin balance between iron ions in the Cu1 and Cu2 layers. Control measurements on other samples of the YBa₂(Cu_{1? x} Fe_x)₃O_7±δ series substantiated these conclusions.     

Few-Baryon Interactions from Lattice QCD

We report the recent progress on the determination of three-nucleon forces (3NF) in lattice Quantum Chromodynamics (QCD). We utilize the Nambu–Bethe–Salpeter wave function to define the potential in quantum field theory, and extract two-nucleon forces and 3NF on equal footing. The enormous computational cost for calculating multi-baryon correlators on the lattice is drastically reduced by developing a novel contraction algorithm (the unified contraction algorithm). Quantum numbers of the three-nucleon system are chosen to be (I, J ^P ) = (1/2, 1/2⁺) (the triton channel), and we extract 3NF in which three nucleons are aligned linearly with an equal spacing. Lattice QCD simulations are performed using N _f = 2 dynamical clover fermion configurations at the lattice spacing of a = 0.156fm on a 16³ × 32 lattice with a large quark mass corresponding to m _π = 1.13 GeV. Repulsive 3NF is found at short distance.     

Thermal production of superheavy magnetic monopoles in the early universe

Starting with an initial abundance of zero, we calculate the thermal production of magnetic monopoles by using the monopole-antimonopole annihilation cross section and detailed balance. The final abundance of monopoles depends exponentially upon m/T_i; m is the monopole mass ～10¹⁶ GeV and T_i is the temperature when production commences. For m/T_i ≈35–55 the present monopole to photon ratio is nM/n_γ≈10^?23?10^?38, less than the bound derived from the present mass density, but possibly large enough to be detected. This calculation provides a lower bound on the monopole abundance for a given m/T_i.     

The speed of sound in hadronic matter

We calculate the speed of sound c _s in an ideal gas of resonances, whose mass spectrum is assumed to have the Hagedorn form ρ(m)～m ^?a exp?{bm}, which leads to singular behavior at the critical temperature T _c =1/b. With a=4 the pressure and the energy density remain finite at T _c , while the specific heat diverges there. As a function of the temperature, the corresponding speed of sound initially increases similarly to that of an ideal pion gas, until near T _c resonance effects dominate, which causes c _s to vanish as (T _c ?T)^1/4. In order to compare this result to the physical resonance gas models, we introduce an upper cut-off M in the resonance mass integration. Although the truncated form still decreases somewhat in the region around T _c , the actual critical behavior in these models is no longer present.     

Effect of lateral tensile stresses on the low-temperature electrical resistivity and magnetoresistance of La0.67Ca0.33MnO3 nanofilms

The thickness of the buffer layer of strontium titanate introduced between an La_0.67Ca_0.33MnO₃ manganite film and a (001)La_0.29Sr_0.71Al_0.65Ta_0.35O₃ substrate is varied (d ₁ = 7–70 nm) to influence effective misfit m in their lattice parameters. As m increases, electrical resistivity ρ of the film increases sharply and the maximum in the ρ(T) dependence shifts toward low temperatures. At T < 150 K, the temperature dependence of ρ of the manganite film obeys the relationship ρ = ρ₁ + ρ₂ T ^4.5, where parameter ρ₁ is independent of the temperature and magnetic field. Coefficient ρ₂ decreases with increasing magnetic field and increases with the misfit between the lattice parameters of the film and substrate, i.e., when the effective hole concentration in the manganite layer decreases.     

Colormagnetic confinement in the quark–gluon thermodynamics

Nonperturbative effects in the quark–gluon thermodynamics are studied in the framework of vacuum correlator method. It is shown, that for T > T ₀ = 175 MeV two correlators: colorelectric D ₁ ^E (x) and colormagnetic D ^H (x), provide the Polyakov line and the colormagnetic confinement in the spatial planes respectively. As a result, both effects produce the realistic behavior of p(T) and I(T), being in good agreement with numerical lattice data.     

Temperature dependence of magnetic anisotropy constant in cobalt ferrite nanoparticles

The temperature dependence of the effective magnetic anisotropy constant K(T) of CoFe₂O₄ nanoparticles is obtained based on the SQUID magnetometry measurements and Mössbauer spectroscopy. The variation of the blocking temperature T_B as a function of particle radius r is first determined by associating the particle size distribution and the anisotropy energy barrier distribution deduced from the hysteresis curve and the magnetization decay curve, respectively. Finally, the magnetic anisotropy constant at each temperature is calculated from the relation between r and T_B. The resultant effective magnetic anisotropy constant K(T) decreases markedly with increasing temperature from 1.1×10⁷ J/m³ at 5 K to 0.6×10⁵ J/m³ at 280 K. The attempt time τ₀ is also determined to be 6.1×10⁻¹² s which together with the K(T) best explains the temperature dependence of superparamagnetic fraction in Mössbauer spectra.     

Urfermionen im kubischen Gitter mit acht Punktlagen

Space is a cubic lattice of points with eight positions in each cell. — This proposition leads, in the limit of an infinite number of points and a vanishing lattice constant, to a Lorentz invariant Schrödingerian for motion if we assume, that only one kind of urfermions exists, and that transitions are possible only to next neighbours. Best adapted to the lattice is an interaction operator which is essentiallyHeisenberg's. In HF-approximation we derive the equations for the masses of quasi particles in the liquid of urfermions. In the now proposed lattice the masses are finite, if and only if the same is true for the interaction constant. We obtain the energyE=±√k ²+m ² and the massm=2.757W even in the limit. It is easy to see, why now we need only a finite value ofW.