New method for studying steady states in quantum impurity problems: the interacting resonant level model

We develop a new perturbative method for studying any steady states of quantum impurities, in or out of equilibrium. We show that steady-state averages are completely fixed by basic properties of the steady-state (Hershfield's) density matrix along with dynamical "impurity conditions." This gives the full perturbative expansion without Feynman diagrams (matrix products instead are used), and "resums" into an equilibrium average that may lend itself to numerical procedures. We calculate the universal current in the interacting resonant level model (IRLM) at finite bias V to first order in Coulomb repulsion U for all V and temperatures. We find that the bias, like the temperature, cuts off low-energy processes. In the IRLM, this implies a power-law decay of the current at large V (also recently observed by Boulat and Saleur at some finite value of U).     

Nonequilibrium transport in quantum impurity models: the Bethe ansatz for open systems

We develop an exact nonperturbative framework to compute steady-state properties of quantum impurities subject to a finite bias. We show that the steady-state physics of these systems is captured by nonequilibrium scattering eigenstates which satisfy an appropriate Lippman-Schwinger equation. Introducing a generalization of the equilibrium Bethe ansatz--the nonequilibrium Bethe ansatz--we explicitly construct the scattering eigenstates for the interacting resonance level model and derive exact, nonperturbative results for the steady-state properties of the system.     

Exact nonequilibrium steady state of a strongly driven open XXZ chain

An exact and explicit ladder-tensor-network ansatz is presented for the nonequilibrium steady state of an anisotropic Heisenberg XXZ spin-1/2 chain which is driven far from equilibrium with a pair of Lindblad operators acting on the edges of the chain only. We show that the steady-state density operator of a finite system of size n is-apart from a normalization constant-a polynomial of degree 2n - 2 in the coupling constant. Efficient computation of physical observables is facilitated in terms of a transfer operator reminiscent of a classical Markov process. In the isotropic case we find cosine spin profiles, 1/n(2) scaling of the spin current, and long-range correlations in the steady state. This is a fully nonperturbative extension of a recent result [Phys. Rev. Lett. 106, 217206 (2011)].     

Stability of quantum states of finite macroscopic systems against classical noises,perturbations from environments,and local measurements

We study the stability of quantum states of macroscopic systems of finite volume V. By using both the locality and huge degrees of freedom, we show the following: (i) If square fluctuation of every additive operator is O(V) or less for a pure state, then it is not fragile for any weak classical noises or weak perturbations from environments. (ii) If square fluctuation of some additive operator is O(V2) for a pure state, then it is fragile for some of these. (iii) If a state, pure or mixed, has the "cluster property," then it is stable against local measurements, and vice versa. Among many applications, we discuss the mechanism of symmetry-breaking in finite systems.     

Variational numerical renormalization group: bridging the gap between NRG and density matrix renormalization group

The numerical renormalization group (NRG) is rephrased as a variational method with the cost function given by the sum of all the energies of the effective low-energy Hamiltonian. This allows us to systematically improve the spectrum obtained by NRG through sweeping. The ensuing algorithm has a lot of similarities to the density matrix renormalization group (DMRG) when targeting many states, and this synergy of NRG and DMRG combines the best of both worlds and extends their applicability. We illustrate this approach with simulations of a quantum spin chain and a single impurity Anderson model where the accuracy of the effective eigenstates is greatly enhanced as compared to the NRG, especially in the transition to the continuum limit.     

Influence of Superconducting Leads Energy Gap on Electron Transport Through Double Quantum Dot by Markovian Quantum Master Equation Approach

The superconducting reservoir effect on the current carrying transport of a double quantum dot in Markovian regime is investigated. For this purpose, a quantum master equation at finite temperature is derived for the many-body density matrix of an open quantum system. The dynamics and the steady-state properties of the double quantum dot system for arbitrary bias are studied. We will show that how the populations and coherencies of the system states are affected by superconducting leads. The energy parameter of system contains essentially four contributions due to dots system-electrodes coupling, intra dot coupling, two quantum dots inter coupling and superconducting gap. The coupling effect of each energy contribution is applied to currents and coherencies results. In addition, the effect of energy gap is studied by considering the amplitude and lifetime of coherencies to get more current through the system.     

Steady-state probability density function in wave turbulence under large volume limit

We investigate the possibility for two-mode probability density function(PDF) to have a non-zero flux steady state solution.We take the large volume limit so that the space of modes becomes continuous.It is shown that in this limit all the steady-state two-or higher-mode PDFs are the product of one-mode PDFs.The flux of this steady-state solution turns out to be zero for any finite mode PDF.     

The exchange interaction effect in the scanning tunneling spectroscopy of a two-orbital Anderson impurity on metallic surface

We investigate the scanning tunneling spectroscopy (STS) of a two-orbital Anderson impurity adsorbed on a metallic surface by using the numerical renormalization group (NRG) method. The density of state of magnetic impurity and the local conduction electron are calculated. We obtain the Fano resonance line shape in the STM conductance at zero temperature. For the impurity atom with antiferromagnetic inter-orbital exchange interaction and a spin singlet ground state, we show that a dip in the STM spectra around zero bias voltage regime and side peaks of spin excitation can be observed. The spin excitation energy is proportional to the exchange interaction strength. As the exchange interaction is ferromagnetic, the underscreened Kondo effect dominates the low energy properties of this system, and it gives rise to drastically different STM spectra as compared with the spin singlet case.     

Exact steady-state density operator for a collective atomic system in an external field

An exact steady-state density operator is obtained for a model describing the collective behaviour of a system of N two-level atoms driven by a classical field. This is used to obtain the exact steady-state expectation value of the atomic population difference for any N.     

Chiral symmetry breaking and the dirac spectrum at nonzero chemical potential

The relation between the spectral density of the QCD Dirac operator at nonzero baryon chemical potential and the chiral condensate is investigated. We use the analytical result for the eigenvalue density in the microscopic regime which shows oscillations with a period that scales as 1/V and an amplitude that diverges exponentially with the volume V = L4. We find that the discontinuity of the chiral condensate is due to the whole oscillating region rather than to an accumulation of eigenvalues at the origin. These results also extend beyond the microscopic regime to chemical potentials mu approximately 1/L.     

简并双光子激光的光子统计

从原子和场模的密度算符的主方程出发,应用Haake和Lewenstein所发展的原子变量绝热消除的算符方法,导出了简并双光子激光光场Wigner函数的福克-普朗克(Fokker-Planck)方程及其稳态解.利用稳态解的高斯近似,求得了在不同泵浦强度下,光子统计的解析结果如数值结果,并与前人的结果作了比较.     

金刚石薄膜负偏压形核的边缘效应

 在微波等离子体化学气相沉积装置中，研究了金刚石薄膜在Si (100)面上的负偏压形核行为，结果表明，偏压大小对金刚石的形核均匀性有显著影响，而甲烷浓度主要影响形核时间，对金刚石的最大核密度影响不大。在硅片尺寸小于钼支撑架时，形核行为存在明显的边缘效应，即在偏压值低于-150 V时，硅片边缘金刚石核密度急剧降低，远低于硅片中央；在甲烷浓度比较低时，硅片边缘核密度要高于中间。研究表明，造成这种现象的主要原因是硅片下的钼支撑架发射电子所致，过量的原子H对金刚石的形核是不利的。     

The effect of low-energy ion bombardment on the density and crystal structure of thin films

The effect of low-energy ion bombardment on the growth and properties of thin films deposited by rf plasma sputtering at low substrate temperatures is studied. The dependences of the film thickness, density, crystal structure, and conductivity on the bias voltage applied to the substrate are obtained. At biases ranging from 0 to −30 V, nickel films are polycrystalline; at higher biases, they exhibit axial (111) texture. At the bias −60 V, the density of the Ni films is close to that of the bulk metal and the crystal structure of the films is the most ordered. With a further increase in the bias, the density of the films drops because of gas (argon and residual gases) atoms incorporated into the films. The same bias dependence of the density is observed for amorphous films of binary alloys of d and f metals. In this case, the films deposited at the substrate bias −40 V have the highest density.     

Microscopic spectrum of the Wilson Dirac operator

We calculate the leading contribution to the spectral density of the Wilson Dirac operator using chiral perturbation theory where volume and lattice spacing corrections are given by universal scaling functions. We find analytical expressions for the spectral density on the scale of the average level spacing, and introduce a chiral random matrix theory that reproduces these results. Our work opens up a novel approach to the infinite-volume limit of lattice gauge theory at finite lattice spacing and new ways to extract coefficients of Wilson chiral perturbation theory.     

非晶金刚石薄膜的X射线反射研究

利用过滤阴极真空电弧系统制备了不同衬底偏压下非晶金刚石薄膜,分别采用X射线反射法测定了相应的非晶金刚石膜密度,分析了薄膜密度与沉积能量之间的变化规律．建立了薄膜密度随衬底偏压的变化曲线。研究发现在-80V时非晶金刚石膜密度存在最大值3．26g／cm^2,随着偏压的增大和减小,薄膜的密度都相应的下降;当衬底偏压加到-2000V时,密度减小到2．63g／cm^2,相对于密度的最大值变化较小。通过薄膜sp^3能态杂化含量与密度的简单比例关系,近似推算出非晶金刚石膜中sp^3能态的含量最高可达80％以上。     

Renormalized quasiparticles in antiferromagnetic states of the Hubbard model

We analyze the properties of the quasiparticle excitations of metallic antiferromagnetic states in a strongly correlated electron system. The study is based on dynamical mean field theory (DMFT) for the infinite dimensional Hubbard model with antiferromagnetic symmetry breaking. Self-consistent solutions of the DMFT equations are calculated using the numerical renormalization group (NRG). The low energy behavior in these results is then analyzed in terms of renormalized quasiparticles. The parameters for these quasiparticles are calculated directly from the NRG derived self-energy, and also from the low energy fixed point of the effective impurity model. From these the quasiparticle weight and the effective mass are deduced. We show that the main low energy features of the k-resolved spectral density can be understood in terms of the quasiparticle picture. We also find that Luttinger's theorem is satisfied for the total electron number in the doped antiferromagnetic state.     

Intrinsic asymmetries in spin-valves: a tight binding model study

We have calculated the I–V curves, dynamical conductance, and tunneling magnetoresistance (TMR) of 1D magnetic tunneling junction through singleband tight binding model calculations based on the non-equilibrium Green's function approach. The difference in density of state of two ferromagnetic leads and the bias dependence of the propagator cause intrinsic asymmetries in TMR and dynamical conductance at finite bias. Besides, we have displayed that large TMR can be obtained even at high bias for half metallic leads.     

A quantum field theory of lattice dynamics at finite temperature

A quantum field theoretical treatment of three-dimensional cubic crystals at finite temperature is presented from the view-point of the spontaneous breakdown of the spatial translational invariance using thermo field theory. The effective interaction Hamiltonian is constructed by taking into account the dynamical map of the molecular density operator which is obtained from the Ward-Takahashi relations. The acoustic phonons are expected to be the excitation of particle-hole pairs. The conventional secular equation for the lattice vibrations is obtained by neglecting some quantum effects in the Bethe-Salpeter equation for the molecular density fluctuations. The phonon spectra, the phonon propagators and the dynamical map of the molecular density operator are calculated at finite temperature.     

A Theoretical Solution for a Nonlinear Master Equation

Based on the master equation describing the interaction of a single-mode bosonic state with a heat bath at finite temperature in the Born-Markov approximation, we constructed a new nonlinear master equation and derived the infinite operator sum representation of quasi-Kraus operators for the density operator     

Ultraviolet photodetector with bandpass characteristic based on a blend of PVK and PBD

We fabricate an ultraviolet photodetector based on a blend of poly (N-vinylcarbazole) (PVK) and 2- tert-butylphenyl-5-biphenyl-1, 3, 4-oxadiazole (PBD) using spin coating. The device exhibites a low dark current density of 2.2×10 3 μA/cm 2 at zero bias. The spectral response of the device shows a narrow bandpass characteristic from 300 to 355 nm, and the peak response is 18.6 mA/W located at 334 nm with a bias of –1 V. We also study the performances of photodetectors with different blend layer thicknesses. The largest photocurrent density is obtained with a blend of 90 nm at the same voltage.