Terahertz time-domain reflection spectroscopy for high-Tc superconducting cuprates

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for reflectivity measurement with a temperature-controllable cryostat. For emission and detection of THz radiation, a Ti:Sapphire pulsed laser and photoconductive antennas are used. Two wire-grid polarizers enable us to carry out the polarized reflectivity measurements. Using our THz-TDS system, we measured the c-axis polarized reflectivity spectra for La_2−xSr_xCuO₄ single crystals (x = 0.10 and 0.13) and observed sharp Josephson plasma edges for both samples below T_c. The reflectivity spectra of the x = 0.10 sample were in good agreement with those reported previously, which confirms the validity of the system. For the x = 0.13 sample, we discuss the anomolous features of the optical spectra.     

Inequivalent quantizations of the rational Calogero model with a Coulomb type interaction

We consider the inequivalent quantizations of a N-body rational Calogero model with a Coulomb type interaction. It is shown that for a certain range of the coupling constants, this system admits a one-parameter family of self-adjoint extensions. We analyze both the bound and scattering state sectors and find novel solutions of this model. We also find the ladder operators for this system, with which the previously found solutions can be constructed.     

Novel approach to angular distributions in precompound reactions Does the Bohr hypothesis always work?

We modify the standard statistical model for precompound reactions (exciton model) by taking into account the correlations between fluctuatingS-matrix elements with differentJ (total spin) values. This is done in the framework of the statistical approach to nuclear reactions. While angle-integrated cross-sections are not affected by our modification, differential cross-sections become asymmetric about 90° c.m. This asymmetry weakens with time and with increasing complexity of the decaying nuclear system, but need not disappear even for the compound (thermalized) system. We present a comparison with data showing such an asymmetry.     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

Nonperturbative treatment of theN-body system with dynamical mesons

The variational approach, used by Feynman in the study of the polaron problem, is generalized to treat a system ofN non-relativistic particles interacting with scalar and vector mesons. After integrating out the meson fields in the path integral formulation we perform a variational calculation for the effectiveN-body two-time action to obtain the energy and the mass of the system. The interplay of self energies and exchange terms in attaining binding and saturation is examined. We estimate the size of the particles in the medium and give the mean number of mesons as a function ofN.Communicated by F. Lenz     

Unconditionally Stable Methods for Hamilton–Jacobi Equations

We present new numerical methods for constructing approximate solutions to the Cauchy problem for Hamilton–Jacobi equations of the form u_t+H(D_xu)=0. The methods are based on dimensional splitting and front tracking for solving the associated (non-strictly hyperbolic) system of conservation laws p_t+D_xH(p)=0, where p=D_xu. In particular, our methods depend heavily on a front tracking method for one-dimensional scalar conservation laws with discontinuous coefficients. The proposed methods are unconditionally stable in the sense that the time step is not limited by the space discretization and they can be viewed as “large-time-step” Godunov-type (or front tracking) methods. We present several numerical examples illustrating the main features of the proposed methods. We also compare our methods with several methods from the literature.     

New Form of the T-Duality Due to the Stability of a Compact Dimension

We study behaviors of a compact dimension and the T-duality, in the presence of the wrapped closed bosonic strings. When the closed strings interact and form another system of strings, the radius of compactification increases. This modifies the T-duality, which we call it as T-duality-like. Some effects of the T-duality-like will be studied.     

外磁场下XY模型中两量子位热纠缠的性质及其调控研究

在海森堡XY模型中,为了统一研究均匀磁场和非均匀磁场对系统热纠缠的影响,在两个量子位分别施加独立可控的外磁场(B+b)和(B-b). 发现在均匀磁场和低温条件下的纠缠度有一个稳定的平台区并发生纠缠突变. 控制磁场不均匀度b和选择合适的材料就可以获得最有利的纠缠,并大大提高系统退纠缠的临界温度T_c. 调节磁场的B值,可以在更宽的温度范围内实现此体系的纠缠开关. 关键词： 热纠缠度 密度矩阵 XY模型')" href="#">XY模型     

Exact results on decoherence and entanglement in a system of N driven atoms and a dissipative cavity mode

We solve the dynamics of an open quantum system where N strongly driven two-level atoms are equally coupled on resonance to a dissipative cavity mode. Analytical results are derived on decoherence, entanglement, purity, atomic correlations and cavity field mean photon number. We predict decoherencefree subspaces for the whole system and the N-qubit subsystem, the monitoring of quantum coherence and purity decay by atomic populations measurements, the conditional generation of atomic multi-partite entangled states and of cavity cat-like states. We show that the dynamics of atoms prepared in states invariant under permutation of any two components remains restricted within the subspace spanned by the completely symmetric Dicke states. We discuss examples and applications in the cases N = 3, 4. An erratum to this article can be found at     

具有长程相互作用S-1/2 XY链的研究

采用平均场Jordan - Wigner 变换分析方法,研究了外场中且具有Z方向均匀长程相互作用自旋－1/2 XY链的热力学性质,得到了系统格点的亥姆赫兹自由能、内能、比热、磁化强度、磁化率等热力学量的解析表达式及其数值解,讨论了系统的一级和两级相变,数值结果在退化条件下与其他文献的结果符合很好. 关键词： XY链')" href="#">XY链 平均场Jordan-Wigner变换 长程相互作用     

The axial form factor in the Nambu-Jona-Lasinio model

We give a general expression for the expectation value of the axial vector currentA _µ ^a in a system with baryon numberB=1, which arises as solution of theSU(2)—flavour Nambu-Jona-Lasinio model in the fermion-1-loop approximation. States with good spin and isospin quantum numbers are constructed by using the lowest order cranking approximation. Taking the self consistent solutions for such a system we calculate the axial form factorg _A(q ²) and the axial coupling constantg _A . The results can be checked by comparing the value of g_A with the one obtained from the radial integral or the asymptotic tail of the self consistent pion field by means of the PCAC relation.     

Nonlinear evolution equations andH theorems

We prove a structure theorem for evolution equations in the state space of a discrete classical system fulfilling a class ofH theorems. TheseH theorems are proved to give strong implications on the time behavior of the solutions. All the results are demonstrated by examples (Boltzmann-like equations, for example).     

Cluster algorithms for anisotropic quantum spin models

We present cluster Monte Carlo algorithms for theXYZ quantum spin models. In the special case ofS=1/2, the new algorithm can be viewed as a cluster algorithm for the 8-vertex model. As an example, we study theS=1/2XY model in two dimensions with a representation in which the quantization axis lies in the easy plane. We find that the numerical autocorrelation time for the cluster algorithm remains of the order of unity and does not show any significant dependence on the temperature, the system size, or the Trotter number. On the other hand, the autocorrelation time for the conventional algorithm strongly depends on these parameters and can be very large. The use of improved estimators for thermodynamic averages further enhances the efficiency of the new algorithms.     

Electron transport through SWNT/<Emphasis Type="Italic">trans</Emphasis>-PA/SWNT structure (the role of solitons): A t-matrix technique

Using a tight-binding model and a transfer-matrix technique, we numerically investigate the effects of the coupling strength and the role of solitons on the electronic transmission through a system in which trans-polyacetylene (trans-PA) molecule is sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT). We rely on Landauer formalism as the basis for studying the conductance properties of this system. Our calculations show that the solitons play an important role in the response of this system causing a large enhancement in the conductance. Also our results suggest that the conductance is sensitive to the CNT/molecule coupling strength.      

Cluster expansion ford-dimensional lattice systems and finite-volume factorization properties

We consider classical lattice systems with finite-range interactions ind dimensions. By means of a block-decimation procedure, we transform our original system into a polymer system whose activity is small provided a suitable factorization property of finite-volume partition functions holds. In this way we extend a result of Olivieri.     

T1–T2 Correlation Spectra Obtained Using a Fast Two-Dimensional Laplace Inversion

Spin relaxation is a sensitive probe of molecular structure and dynamics. Correlation of relaxation time constants, such as T₁ and T₂, conceptually similar to the conventional multidimensional spectroscopy, have been difficult to determine primarily due to the absense of an efficient multidimensional Laplace inversion program. We demonstrate the use of a novel computer algorithm for fast two-dimensional inverse Laplace transformation to obtain T₁–T₂ correlation functions. The algorithm efficiently performs a least-squares fit on two-dimensional data with a nonnegativity constraint. We use a regularization method to find a balance between the residual fitting errors and the known noise amplitude, thus producing a result that is found to be stable in the presence of noise. This algorithm can be extended to include functional forms other than exponential kernels. We demonstrate the performance of the algorithm at different signal-to-noise ratios and with different T₁–T₂ spectral characteristics using several brine-saturated rock samples.     

Quenching dynamics of a quantum XY spin-1/2 chain in the presence of transverse field by the application of a generalized Landau-Zener formula

In this paper we review the quenching dynamics of a quantum XY spin-1/2 chain in the presence of a transverse field, when the transverse field or the anisotropic interaction is quenched at a slow but uniform rate. We also extend the results to the cases in which the system starts with any arbitrary initial condition as opposed to the initial fully magnetically aligned state which has been extensively studied earlier. The evolution is non-adiabatic in the time interval when the parameters are close to their critical values, and is adiabatic otherwise. The density of defects produced due to nonadiabatic transitions is calculated by mapping the many-particle system to an equivalent Landau-Zener problem. We show that in one dimension the density of defects in the final state scales as 1/√τ irrespective of the initial condition, where τ is the quenching time-scale. However, the magnitude of density of defects is found to depend on the initial condition.