Publications (25)83.27 Total impact
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ABSTRACT: Spin susceptibility of Anderson impurities is a key quantity in understanding the physics of Kondo screening. Traditional numerical renormalization group (NRG) calculation of the impurity contribution $\chi_{\textrm{imp}}$ to susceptibility, defined originally by Wilson in a flat wide band, has been generalized to structured conduction bands. The results brought about nonFermiliquid and diamagnetic Kondo behaviors in $\chi_{\textrm{imp}}$, even when the bands are not gapped at the Fermi energy. Here we use the full densitymatrix (FDM) NRG to present highquality data for the local susceptibility $\chi_{\textrm{loc}}$ and to compare them with $\chi_{\textrm{imp}}$ obtained by the traditional NRG. Our results indicate that those exotic behaviors observed in $\chi_{\textrm{imp}}$ are unphysical. Instead, the lowenergy excitations of the impurity in arbitrary bands only without gap at the Fermi energy are still a Fermi liquid and paramagnetic. We also demonstrate that unlike the traditional NRG yielding less accurate $\chi_{\textrm{loc}}$ than $\chi_{\textrm{imp}}$, the FDM method allows a highprecision dynamical calculation of $\chi_{\textrm{loc}}$ at much reduced computational cost, with an accuracy at least one order higher than $\chi_{\textrm{imp}}$. Moreover, artifacts in the FDM algorithm to $\chi_{\textrm{imp}}$, and origins of the spurious nonFermiliquid and diamagnetic features are clarified. Our work provides an efficient highprecision way to calculate the susceptibility of impurity for arbitrary structured bands, while negating the applicability of Wilson's definition to such cases. 
Article: A Standard Basis Operator Equation of Motion Impurity Solver for Dynamical Mean Field Theory
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ABSTRACT: We present an efficient impurity solver for the dynamical meanfield theory (DMFT). It is based on the separation of bath degrees of freedom into the low energy and the high energy parts. The former is solved exactly using exact diagonalization and the latter is treated approximately using Green's function equation of motion decoupling approximation. The two parts are combined coherently under the standard basis operator formalism. The impurity solver is applied to the Anderson impurity model and, combined with DMFT, to the oneband Hubbard model. Qualitative agreement is found with other well established methods. Some promising features and possible improvements of the present solver are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a Monte Carlo algorithm for the free energy calculation based on configuration space sampling. An upward or downward temperature scan can be used to produce F(T). We implement this algorithm for the Ising model on a square lattice and triangular lattice. Comparison with the exact free energy shows an excellent agreement. We analyze the properties of this algorithm and compare it with the WangLandau algorithm, which samples in energy space. This method is applicable to general classical statistical models. The possibility of extending it to quantum systems is discussed.  [Show abstract] [Hide abstract]
ABSTRACT: We study the spin1/2 J1J2 Heisenberg model on a square lattice using the cluster meanfield theory. We find a rapid convergence of phase boundaries with increasing cluster size. By extrapolating the cluster size L to infinity, we obtain accurate phase boundaries [Formula: see text] (between the Néel antiferromagnetic phase and nonmagnetic phase), and [Formula: see text] (between nonmagnetic phase and the collinear antiferromagnetic phase). Our results support the secondorder phase transition at [Formula: see text] and the firstorder one at [Formula: see text]. For the spinanisotropic J1J2 model, we present its finite temperature phase diagram and demonstrate that the nonmagnetic state is unstable towards the firstorder phase transition under intermediate spin anisotropy.  [Show abstract] [Hide abstract]
ABSTRACT: The quantum Fisher information characterizes the phase sensitivity of qubits in the spinboson model with a finite bandwidth spectrum. In contrast with Markovian reservoirs, the quantum Fisher information flows from the environments to qubits after a number of times if the bath parameter s is larger than a critical value which is related to temperature. The suddenchange behavior will happen during the evolution of the quantum Fisher information of the maximal entanglement state in the nonMarkovian environments. The suddenchange times can be varied with the change of the bath parameter s. For a very large number of entangled qubits, the suddenchange behavior of the maximal quantum Fisher information can be used to characterize the existence of the entanglement. The metrology strategy based on the quantum correlated state leads to a lower phase uncertainty when compared with the uncorrelated product state.  [Show abstract] [Hide abstract]
ABSTRACT: We study the critical behavior of the singlesite entanglement entropy S at the Mott metalinsulator transition in infinitedimensional Hubbard model. For this model, the entanglement between a single site and rest of the lattice can be evaluated exactly, using the dynamical meanfield theory (DMFT). Both the numerical solution using exact diagonalization and the analytical one using twosite DMFT gives SSc \propto \alpha \log_{2}[(1/2Dc)/Dc](UUc), with Dc the double occupancy at Uc and \alpha < 0 being different on two sides of the transition. 
Article: Simulation of the spinboson model with superconducting phase qubit coupled to a transmission line
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ABSTRACT: Based on the rapid experimental developments of circuit QED, we propose a feasible scheme to simulate a spinboson model with the superconducting circuits, which can be used to detect quantum KosterlitzThouless (KT) phase transition. We design the spinboson model by using a superconducting phase qubit coupled with a semiinfinite transmission line, which is regarded as bosonic reservoir with a continuum spectrum. By tuning the bias current or the coupling capacitance, the quantum KT transition can be directly detected through tomography measurement on the states of the phase qubit. We also estimate the experimental parameters using numerical renormalization group method. Comment: 4 pages 
Article: Scaling Analysis in the Numerical Renormalization Group Study of the SubOhmic SpinBoson Model
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ABSTRACT: The spinboson model has nontrivial quantum phase transitions in the subOhmic regime. For the bath spectra exponent $0 \leqslant s<1/2$, the bosonic numerical renormalization group (BNRG) study of the exponents $\beta$ and $\delta$ are hampered by the boson state truncation which leads to artificial interacting exponents instead of the correct Gaussian ones. In this paper, guided by a meanfield calculation, we study the order parameter function $m(\tau=\alpha\alpha_c, \epsilon, \Delta)$ using BNRG. Scaling analysis with respect to the boson state truncation $N_{b}$, the logarithmic discretization parameter $\Lambda$, and the tunneling strength $\Delta$ are carried out. Truncationinduced multiplepower behaviors are observed close to the critical point, with artificial values of $\beta$ and $\delta$. They cross over to classical behaviors with exponents $\beta=1/2$ and $\delta=3$ on the intermediate scales of $\tau$ and $\epsilon$, respectively. We also find $\tau/\Delta^{1s}$ and $\epsilon/\Delta$ scalings in the function $m(\tau, \epsilon, \Delta)$. The role of boson state truncation as a scaling variable in the BNRG result for $0 \leqslant s<1/2$ is identified and its interplay with the logarithmic discretization revealed. Relevance to the validity of quantumtoclassical mapping in other impurity models is discussed.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the interacting Dirac fermions on honeycomb lattice by cluster dynamical meanfield theory (CDMFT) combined with continuous time quantum Monte Carlo simulation (CTQMC). A novel scenario for the semimetalMott insulator transition of the interacting Dirac fermions is found beyond the previous DMFT studies. We demonstrate that the nonlocal spatial correlations play a vital role in the Mott transition on the honeycomb lattice. We also elaborate the experimental protocol to observe this phase transition by the ultracold atoms on optical honeycomb lattice. Comment: 4 pages, 6 figures  [Show abstract] [Hide abstract]
ABSTRACT: The effect of doping in the twodimensional Hubbard model is studied within finitetemperature exact diagonalization combined with cluster dynamical meanfield theory. By employing a mixed basis involving cluster sites and bath molecular orbitals for the projection of the lattice Green’s function onto 2×2 clusters, a considerably more accurate description of the lowfrequency properties of the selfenergy is achieved than in a pure site picture. To evaluate the phase diagram, the transition from Fermiliquid to nonFermiliquid behavior for decreasing hole doping is studied as a function of Coulomb energy, nextnearestneighbor hopping, and temperature. The selfenergy component ΣX associated with X=(π,0) is shown to develop a collective mode above EF, whose energy and strength exhibits a distinct dispersion with doping. This lowenergy excitation gives rise to nonFermiliquid behavior as the hole doping decreases below a critical value δc, and to an increasing particlehole asymmetry, in agreement with recent photoemission data. This behavior is consistent with the removal of spectral weight from electron states above EF and the opening of a pseudogap, which increases with decreasing doping. The phase diagram reveals that δc≈0.15…0.20 for various system parameters. For electron doping, the collective mode of ΣX(ω) and the concomitant pseudogap are located below the Fermi energy, which is consistent with the removal of spectral weight from the hole states just below EF. The critical doping, which marks the onset of nonFermiliquid behavior, is systematically smaller than for hole doping.  [Show abstract] [Hide abstract]
ABSTRACT: The effect of doping in the twodimensional Hubbard model is studied within finite temperature exact diagonalization combined with cluster dynamical mean field theory. By employing a mixed basis involving cluster sites and bath molecular orbitals for the projection of the lattice Green's function onto 2x2 clusters, a considerably more accurate description of the low frequency properties of the selfenergy is achieved than in a pure site picture. The transition from Fermiliquid to nonFermiliquid behavior for decreasing hole doping is studied as a function of Coulomb energy, nextnearest neighbor hopping, and temperature. In particular, the selfenergy component Sigma_X associated with X=(pi,0) is shown to exhibit an onset of nonFermiliquid behavior as the hole doping decreases below a critical value delta_c. The imaginary part of Sigma_X(omega) then develops a collective mode above E_F, which exhibits a distinct dispersion with doping. Accordingly, the real part of Sigma_X(omega) has a positive slope above E_F, giving rise to an increasing particlehole asymmetry as the system approaches the Mott transition. This behavior is consistent with the removal of spectral weight from electron states close to E_F and the opening of a pseudogap which increases with decreasing doping. The phase diagram reveals that delta_c = 0.15 ... 0.20 for various system parameters. For electron doping, the collective mode of Sigma_X(omega) and the concomitant pseudogap are located below the Fermi energy which is consistent the removal of spectral weight from hole states just below E_F. The critical doping which marks the onset of nonFermiliquid behavior, is systematically smaller than for hole doping. Comment: 18 pages, 21 figures  [Show abstract] [Hide abstract]
ABSTRACT: The dynamical mean field theory (DMFT), which is successful in the study of strongly correlated fermions, was recently extended to boson systems [Phys. Rev. B {\textbf 77}, 235106 (2008)]. In this paper, we employ the bosonic DMFT to study the BoseHubbard model which describes onsite interacting bosons in a lattice. Using exact diagonalization as the impurity solver, we get the DMFT solutions for the Green's function, the occupation density, as well as the condensate fraction on a Bethe lattice. Various phases are identified: the Mott insulator, the BoseEinstein condensed (BEC) phase, and the normal phase. At finite temperatures, we obtain the crossover between the Mottlike regime and the normal phase, as well as the BECtonormal phase transition. Phase diagrams on the $\mu/U\tilde{t}/U$ plane and on the $T/U\tilde{t}/U$ plane are produced ($\tilde{t}$ is the scaled hopping amplitude). We compare our results with the previous ones, and discuss the implication of these results to experiments. Comment: 11 pages, 8 figures 
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ABSTRACT: The complete strain tensor in the tiltedgrown epilayer is studied based on the linear elastic theory. With the boundary constrain conditions, the tilt angle of an epilayer can be obtained from the minimization of the strain energy. In this way we can also describe the distortion of crystal cells in the epilayer. In this Letter, as an application of our theory, we focus on the growth of the MnAs epilayer on the GaAs(001) substrate. It is shown that the lattice mismatch strain between the MnAs epilayer and the GaAs substrate can be relaxed by two mechanisms. On the one hand, by forming the lattice coincidence construction at the interface, the type A growth can be realized. On the other hand, by tilting the epilayer by about 30° with respect to the substrate, the type B growth is favored. The competition between the two mechanisms is near an equilibrium for this specific system, and depending on the growth conditions, it may lead to either an A type growth or a B type growth. Our theoretical results agree well with the reported experimental observations.  [Show abstract] [Hide abstract]
ABSTRACT: We study a mesoscopic ring with an inline quantum dot threaded by an AharonovBohm flux. Zeropoint fluctuations of the electromagnetic environment capacitively coupled to the ring, with omega(s) spectral density, can suppress tunneling through the dot, resulting in a quantum phase transition from an unpolarized to a polarized phase. We show that robust signatures of such a transition can be found in the response of the persistent current in the ring to the external flux as well as to the bias between the dot and the arm. Particular attention is paid to the experimentally relevant cases of Ohmic (s = 1) and subOhmic (s = 1/2) noise.  [Show abstract] [Hide abstract]
ABSTRACT: We present a detailed model study of exciton transfer processes in donor–bridge–acceptor (DBA) systems. Using a model which includes the intermolecular Coulomb interaction and the coupling to a dissipative environment we calculate the phase diagram, the absorption spectrum as well as dynamic equilibrium properties with the numerical renormalization group. This method is nonperturbative and therefore allows one to cover the full parameter space, especially the case when the intermolecular Coulomb interaction is of the same order as the coupling to the environment and perturbation theory cannot be applied. For DBA systems with up to six sites we found a transition to the localized phase (selftrapping) depending on the coupling to the dissipative environment. We discuss various criteria which favour delocalized exciton transfer.  [Show abstract] [Hide abstract]
ABSTRACT: The variational cluster approximation (VCA) proposed by M. Potthoff et al. Phys. Rev. Lett. 91 206402 (2003) is extended to electron or spin systems with nonlocal interactions. By introducing more than one source field in the action and employing the Legendre transformation, we derive a generalized selfenergy functional with stationary properties. Applying this functional to a proper reference system, we construct the extended VCA (EVCA). In the limit of continuous degrees of freedom for the reference system, EVCA can recover the cluster extension of the extended dynamical meanfield theory (EDMFT). For a system with correlated hopping, the EVCA recovers the cluster extension of the dynamical meanfield theory for correlated hopping. Using a discrete reference system composed of decoupled threesite single impurities, we test the theory for the extended Hubbard model. Quantitatively good results as compared with EDMFT are obtained. We also propose VCA (EVCA) based on clusters with periodic boundary conditions. It has the (extended) dynamical cluster approximation as the continuous limit. A number of related issues are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate electron transfer processes in donoracceptor systems with a coupling of the electronic degrees of freedom to a common bosonic bath. The model allows to study manyparticle effects and the influence of the local Coulomb interaction U between electrons on donor and acceptor sites. Using the nonperturbative numerical renormalization group approach we find distinct differences between the electron transfer characteristics in the single and twoparticle subspaces. We calculate the critical electronboson coupling alpha_c as a function of $U$ and show results for densitydensity correlation functions in the whole parameter space. The possibility of manyparticle (bipolaronic) and Coulombassisted transfer is discussed.  [Show abstract] [Hide abstract]
ABSTRACT: The effective theories for many quantum phase transitions can be mapped onto those of classical transitions. Here we show that the naive mapping fails for the subOhmic spinboson model which describes a twolevel system coupled to a bosonic bath with powerlaw spectral density, J(omega) proportional, variantomega(s). Using an epsilon expansion we prove that this model has a quantum transition controlled by an interacting fixed point at small s, and support this by numerical calculations. In contrast, the corresponding classical longrange Ising model is known to display meanfield transition behavior for 0 < s < 1/2, controlled by a noninteracting fixed point. The failure of the quantumclassical mapping is argued to arise from the longranged interaction in imaginary time in the quantum model.  [Show abstract] [Hide abstract]
ABSTRACT: We present a detailed description of the recently proposed numerical renormalization group method for models of quantum impurities coupled to a bosonic bath. Specifically, the method is applied to the spinboson model, both in the Ohmic and subOhmic cases. We present various results for static as well as dynamic quantities and discuss details of the numerical implementation, e.g., the discretization of a bosonic bath with arbitrary continuous spectral density, the suitable choice of a finite basis in the bosonic Hilbert space, and questions of convergence w.r.t. truncation parameters. The method is shown to provide highaccuracy data over the whole range of model parameters and temperatures, which are in agreement with exact results and other numerical data from the literature. Comment: 23 pages, 21 figures; three references and one figure added
Publication Stats
618  Citations  
83.27  Total Impact Points  
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Institutions

20062015

Renmin University of China
 Department of Physics
Peping, Beijing, China


20032005

Universität Augsburg
 Institute of Physics
Augsberg, Bavaria, Germany


20002001

Chinese Academy of Sciences
 • State Key Laboratory of Magnetism
 • Condensed Matter Physics
Beijing, Beijing Shi, China
