Magnetoresistance anisotropy of a one-dimensional superconducting niobium strip

We investigated confinement effects on the resistive anisotropy of a superconducting niobium strip with a rectangular cross section. When its transverse dimensions are comparable to the superconducting coherence length, the angle dependent magnetoresistances at a fixed temperature can be scaled as R(theta,H) = R(H/Hctheta) where Hctheta =Hc0(cos2theta + gamma(-2)sin2theta)(-1/2) is the angular dependent critical field, gamma is the width to thickness ratio, and Hc0 is the critical field in the thickness direction at theta=0 degrees . The results can be understood in terms of the anisotropic diamagnetic energy for a given field in a one-dimensional superconductor.     

Microcanonical approach to the simulation of first-order phase transitions

A simple microcanonical strategy for the simulation of first-order phase transitions is proposed. At variance with flat-histogram methods, there is no iterative parameters optimization nor long waits for tunneling between the ordered and the disordered phases. We test the method in the standard benchmark: the Q-states Potts model (Q=10 in two dimensions and Q=4 in D=3). We develop a cluster algorithm for this model, obtaining accurate results for systems with more than 10(6) spins.     

Exact solutions of the D-dimensional Schrödinger equation for a ring-shaped pseudoharmonic potential

A new non-central potential, consisting of a pseudoharmonic potential plus another recently proposed ring-shaped potential, is solved. It has the form $ V(r,\theta ) = \tfrac{1} {8}\kappa r_e^2 \left( {\tfrac{r} {{r_e }} - \tfrac{{r_e }} {r}} \right)^2 + \tfrac{{\beta cos^2 \theta }} {{r^2 sin^2 \theta }} A new non-central potential, consisting of a pseudoharmonic potential plus another recently proposed ring-shaped potential, is solved. It has the form . The energy eigenvalues and eigenfunctions of the bound-states for the Schr?dinger equation in D-dimensions for this potential are obtained analytically by using the Nikiforov-Uvarov method. The radial and angular parts of the wave functions are obtained in terms of orthogonal Laguerre and Jacobi polynomials. We also find that the energy of the particle and the wave functions reduce to the energy and the wave functions of the bound-states in three dimensions.      

Abelian 2-form gauge theory: superfield formalism

We derive the off-shell nilpotent Becchi–Rouet–Stora–Tyutin (BRST) and anti-BRST symmetry transformations for all the fields of a free Abelian 2-form gauge theory by exploiting the geometrical superfield approach to the BRST formalism. The above four (3+1)-dimensional (4D) theory is considered on a (4, 2)-dimensional supermanifold parameterized by the four even spacetime variables x ^μ (with μ=0,1,2,3) and a pair of odd Grassmannian variables θ and (with ). One of the salient features of our present investigation is that the above nilpotent (anti-) BRST symmetry transformations turn out to be absolutely anticommuting due to the presence of a Curci–Ferrari (CF) type of restriction. The latter condition emerges due to the application of our present superfield formalism. The actual CF condition, as is well known, is the hallmark of a 4D non-Abelian 1-form gauge theory. We demonstrate that our present 4D Abelian 2-form gauge theory imbibes some of the key signatures of the 4D non-Abelian 1-form gauge theory. We briefly comment on the generalization of our superfield approach to the case of Abelian 3-form gauge theory in four, (3+1), dimensions of spacetime.     

Multifractality and conformal invariance at 2D metal-insulator transition in the spin-orbit symmetry class

We study the multifractality (MF) of critical wave functions at boundaries and corners at the metal-insulator transition (MIT) for noninteracting electrons in the two-dimensional (2D) spin-orbit (symplectic) universality class. We find that the MF exponents near a boundary are different from those in the bulk. The exponents at a corner are found to be directly related to those at a straight boundary through a relation arising from conformal invariance. This provides direct numerical evidence for conformal invariance at the 2D spin-orbit MIT. The presence of boundaries modifies the MF of the whole sample even in the thermodynamic limit.     

Phase transitions and critical phenomena in a three-dimensional site-diluted Potts model

The phase transitions and critical phenomena in the three-dimensional (3D) site-diluted q-state Potts models on a simple cubic lattice are explored. We systematically study the phase transitions of the models for q=3 and q=4 on the basis of Wolff high-effective algorithm by the Monte–Carlo (MC) method. The calculations are carried out for systems with periodic boundary conditions and spin concentrations p=1.00–0.65. It is shown that introducing of weak disorder (p∼0.95) into the system is sufficient to change the first order phase transition into a second order one for the 3D 3-state Potts model, while for the 3D 4-state Potts model, such a phase transformation occurs when introducing strong disorder (p∼0.65). Results for 3D pure 3-state and 4-state Potts models (p=1.00) agree with conclusions of mean field theory. The static critical exponents of the specific heat α, susceptibility γ, magnetization β, and the exponent of the correlation radius ν are calculated for the samples on the basis of finite-size scaling theory.     

Study of CP(N-1) theta-vacua by cluster simulation of SU(N) quantum spin ladders

D-theory provides an alternative lattice regularization of the 2D CP(N-1) quantum field theory in which continuous classical fields emerge from the dimensional reduction of discrete SU(N) quantum spins. Spin ladders consisting of n transversely coupled spin chains lead to a CP(N-1) model with a vacuum angle theta=npi. In D-theory no sign problem arises and an efficient cluster algorithm is used to investigate theta-vacuum effects. At theta=pi there is a first order phase transition with spontaneous breaking of charge conjugation symmetry for CP(N-1) models with N>2.     

Higher Conformal Multifractality

We derive, from conformal invariance and quantum gravity, the multifractal spectrum f() of the harmonic measure (i.e., electrostatic potential, or diffusion field) near any conformally invariant fractal in two dimensions. It gives the Hausdorff dimension of the set of points where the potential varies with distance r to the fractal frontier as r . First examples are a random walk, i.e., a Brownian motion, a self-avoiding walk, or a critical percolation cluster. The generalized dimensions D(n) as well as the multifractal functions f() are derived, and are all identical for these three cases. The external frontiers of a Brownian motion and of a percolation cluster are thus identical to a self-avoiding walk in the scaling limit. The multifractal (MF) function f(,c) of the electrostatic potential near any conformally invariant fractal boundary, like a critical O(N) loop or a Q-state Potts cluster, is given as a function of the central charge c of the associated conformal field theory. The dimensions D _EP of the external perimeter and D _H of the hull of a critical scaling curve or cluster obey the superuniversal duality equation . Finally, for a conformally invariant scaling curve which is simple, i.e., without double points, we derive higher multifractal functions, like the universal function f ₂(,) which gives the Hausdorff dimension of the points where the potential varies jointly with distance r as r on one side of the curve, and as r on the other. The general case of the potential distribution between the branches of a star made of an arbitrary number of scaling paths is also treated. The results apply to critical O(N) loops, Potts clusters, and to the SLE process. We present a duality between external perimeters of Potts clusters and O(N) loops at their critical point, as well as the corresponding duality in the SLE process for =16.     

Aging in 1D discrete spin models and equivalent systems

We derive exact expressions for a number of aging functions that are scaling limits of nonequilibrium correlations, R(t(w),t(w)+t) as t(w)-->infinity, t/t(w)-->theta, in the 1D homogenous q-state Potts model for all q with T = 0 dynamics following a quench from T = infinity. One such quantity is (0)(t(w));sigma-->(n)(t(w)+t)> when n/square root of ([t(w))-->z. Exact, closed-form expressions are also obtained when an interlude of T = infinity dynamics occurs. Our derivations express the scaling limit via coalescing Brownian paths and a "Brownian space-time spanning tree," which also yields other aging functions, such as the persistence probability of no spin flip at 0 between t(w) and t(w)+t.     

Novel magnetism in 3He nanoclusters

The magnetic susceptibility of 3He nanoclusters embedded in a 4He matrix has been measured from 0.5 to 10 mK at pressures from 2.88 to 3.54 MPa. Even the lowest pressure clusters have a solid fraction in the region of the phase diagram where bulk solid is unstable. At 3.54 MPa, straight theta = -250 microK, equal to that of bulk 3He for v = 21.3 cm3/mole. For 2.88 MPa, straight theta = 140 microK, indicating a ferromagnetic tendency, similar to 2D films at some coverages. At intermediate pressures, chi has a peak near 1.05 mK, but with no discontinuity. Magnetic ordering in nanoclusters appears to be different than the U2D2 phase of bulk 3He.     

Unique spin dynamics and unconventional superconductivity in the layered heavy fermion compound CeIrIn5: NQR evidence

We report measurements of the 115In nuclear spin-lattice relaxation rate ( 1/T1) between T = 0.09 and 100 K in the new heavy fermion (HF) compound CeIrIn5. At 0.4 < or = T< or = 100 K, 1/T1 is strongly T-dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that 1/T1T, subtracting that for LaIrIn5, follows a (1 / T+straight theta)3/4 variation with straight theta = 8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T1 follows a T3 variation, which suggests unconventional superconductivity with line-node gap.     

Linking numbers for self-avoiding loops and percolation: application to the spin quantum hall transition

Nonlocal twist operators are introduced for the O(n) and Q-state Potts models in two dimensions which count the numbers of self-avoiding loops (respectively, percolation clusters) surrounding a given point. Their scaling dimensions are computed exactly. This yields many results: for example, the number of percolation clusters which must be crossed to connect a given point to an infinitely distant boundary. Its mean behaves as (1/3sqrt[3] pi) |ln( p(c)-p)| as p-->p(c)-. As an application we compute the exact value sqrt[3]/2 for the conductivity at the spin Hall transition, as well as the shape dependence of the mean conductance in an arbitrary simply connected geometry with two extended edge contacts.     

Monte Carlo study of the phase transitions in 2D ferro- and antiferromagnetic Potts models on a triangular lattice

The phase transitions in 2D ferro- and antiferromagnetic Potts models with number of spin states q = 3 on a triangular lattice are investigated by the cluster and classical Monte Carlo methods. Systems with linear sizes L = 20–120 are considered. Fourth-order Binder cumulants and histogram data analysis are used to show that second- and first-order phase transitions are observed in the ferromagnetic and antiferromagnetic Potts models, respectively. The static critical indices are calculated for specific heat α, susceptibility γ, magnetization β, and correlation length ν on the basis of finite-size scaling theory for a ferromagnetic Potts model.     

Phase transition in the three-state Potts antiferromagnet on the diced lattice

We prove that the 3-state Potts antiferromagnet on the diced lattice (dual of the kagome lattice) has entropically driven long-range order at low temperatures (including zero). We then present Monte Carlo simulations, using a cluster algorithm, of the 3-state and 4-state models. The 3-state model has a phase transition to the high-temperature disordered phase at v=e;{J}-1=-0.860 599+/-0.000 004 that appears to be in the universality class of the 3-state Potts ferromagnet. The 4-state model is disordered throughout the physical region, including at zero temperature.     

The Rotational Spectra, Structure, Internal Dynamics, and Electric Dipole Moment of the Argon-Ketene van der Waals Complex

Pulsed-beam Fourier transform microwave spectroscopy was used to observe and assign the rotational spectra of the argon-ketene van der Waals complex. Tunneling of the hydrogen or deuterium atoms splits the a- and b-type rotational transitions of H(2)CCO-Ar, H(2)(13)CCO-Ar, H(2)C(13)CO-Ar, and D(2)CCO-Ar into two states. This internal motion appears to be quenched for HDCCO-Ar where only one state is observed. The spectra of all isotopomers were satisfactorily fit to a Watson asymmetric top Hamiltonian which gave A=10 447.9248(10) MHz, B=1918.0138(16) MHz, C=1606.7642(15) MHz, Delta(J)=16.0856(70) kHz, Delta(JK)=274.779(64) kHz, Delta(K)=-152.24(23) kHz, delta(J)=2.5313(18) kHz, delta(K)=209.85(82) kHz, and h(K)=1.562(64) kHz for the A(1) state of H(2)CCO-Ar. Electric dipole moment measurements determined &mgr;(a)=0.417(10)x10(-30) C m [0.125(3) D] and &mgr;(b)=4.566(7)x10(-30) C m [1.369(2) D] along the a and b principal axes of the A(1) state of the normal isotopomer. A least squares fit of principal moments of inertia, I(a) and I(c), of H(2)CCO-Ar, H(2)(13)CCO-Ar, and H(2)C(13)CO-Ar for the A(1) states give the argon-ketene center of mass separation, R(cm)=3.5868(3) ?, and the angle between the line connecting argon with the center of mass of ketene and the C=C=O axis, θ(cm)=96.4 degrees (2). The spectral data are consistent with a planar geometry with the argon atom tilted toward the carbonyl carbon of ketene by 6.4 degrees from a T-shaped configuration. Copyright 2001 Academic Press.     

A numerical method to compute exactly the partition function with application toZ(n) theories in two dimensions

I present a new method to exactly compute the partition function of a class of discrete models in arbitrary dimensions. The time for the computation for ann-state model on anL ^d lattice scales like . I show examples of the use of this method by computing the partition function of the 2D Ising and 3-state Potts models for maximum lattice sizes 10×10 and 8×8, respectively. The critical exponentsv and and the critical temperature one obtains from these are very near the exactly known values. The distribution of zeros of the partition function of the Potts model leads to the conjecture that the ratio of the amplitudes of the specific heat below and above the critical temperature is unity.     

Spatial persistence of fluctuating interfaces

We show that the probability, P0(l), that the height of a fluctuating (d+1)-dimensional interface in its steady state stays above its initial value up to a distance l, along any linear cut in the d-dimensional space, decays as P0(l) approximately l(theta). Here straight theta is a "spatial" persistence exponent, and takes different values, straight theta(s) or straight theta(0), depending on how the point from which l is measured is specified. These exponents are shown to map onto corresponding temporal persistence exponents for a generalized d = 1 random-walk equation. The exponent straight theta(0) is nontrivial even for Gaussian interfaces.     

Persistence in systems with algebraic interaction

Persistence in coarsening one-dimensional spin systems with a power-law interaction r(-1-sigma) is considered. Numerical studies indicate that for sufficiently large values of the interaction exponent sigma (sigma > or =1/2 in our simulations), persistence decays as an algebraic function of the length scale L, P(L) approximately L(-theta). The persistence exponent theta is found to be independent on the force exponent sigma and close to its value for the extremal (sigma-->infinity) model, theta =0.175 075 88. For smaller values of the force exponent (sigma < 1/2), finite size effects prevent the system from reaching the asymptotic regime. Scaling arguments suggest that in order to avoid significant boundary effects for small sigma, the system size should grow as [O(1/sigma)](1/sigma).     

Quasifree compton scattering from the deuteron and nucleon polarizabilities

Cross sections for quasifree Compton scattering from the deuteron were measured for incident energies of E(gamma) = 236-260 MeV at the laboratory angle straight theta(gamma(')) = -135 degrees. The recoil nucleons were detected in a liquid-scintillator array situated at straight theta(N) = 20 degrees. The measured differential cross sections were used, with the calculations of Levchuk et al., to determine the polarizabilities of the bound nucleons. For the bound proton, the extracted values were consistent with the accepted value for the free proton. Combining our results for the bound neutron with those from Rose et al., we obtain 1-sigma constraints of alpha;(n) = 7.6-14.0 and beta;(n) = 1.2-7.6.     

Mikheyev-smirnov-wolfenstein effects in vacuum oscillations

We point out that for solar neutrino oscillations with the mass-squared difference of Deltam(2) approximately 10(-10)-10(-9) eV(2), i.e., in the so-called vacuum oscillation range, the solar matter effects are non-negligible, particularly for the low energy pp neutrinos. One consequence of this is that the values of the mixing angle straight theta and pi/2-straight theta are not equivalent, making it necessary to consider the entire physical range of the mixing angle 0相似文献