Publications (20)78.38 Total impact

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 pagesScience China: Physics, Mechanics and Astronomy 12/2010; · 1.17 Impact Factor 
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.Physical review. B, Condensed matter 12/2010; · 3.77 Impact Factor  [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 figuresPhysical review. B, Condensed matter 05/2010; · 3.77 Impact Factor  [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 figures07/2009;  [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 figuresPhysical review. B, Condensed matter 07/2009; · 3.77 Impact Factor  Physical Review Letters 06/2009; 102(24). · 7.73 Impact Factor
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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.Physical review. B, Condensed matter 01/2009; 80(16). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
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.Physics Letters A  PHYS LETT A. 01/2007; 367(4):373378.  [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.Physical Review Letters 08/2006; 97(1):016802. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a detailed model study of exciton transfer processes in donorbridgeacceptor (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.Journal of Physics Condensed Matter 07/2006; 18(26):59856000. · 2.22 Impact Factor  [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.Physical Review B 04/2005; 72(11). · 3.66 Impact Factor  [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.EPL (Europhysics Letters) 03/2005; · 2.26 Impact Factor  [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.Physical Review Letters 02/2005; 94(7):070604. · 7.73 Impact Factor  [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 addedPhysical review. B, Condensed matter 07/2004; · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the extended Hubbard model with both onsite (U) and nearest neighbor (V) Coulomb repulsion using the exact diagonalization method within the dynamical mean field theory. For a fixed U (U=2.0), the Tn phasediagrams are obtained for V=1.4 and V=1.2, at which the ground states of n=1/2 system is chargeordered and chargedisordered, respectively. In both cases, robust charge order is found at finite temperature and in an extended filling regime around n=1/2. The order parameter changes nonmonotonously with temperature. For V=1.4, phase separation between chargeordered and chargedisordered phases is observed in the low temperature and n < 0.5 regime. It is described by an "S"shaped structure of the n/mu curve. For V=1.2, the ground state is chargedisordered, and a reentrant chargeordering transition is observed for 0.42 < n < 0.68. Relevance of our results to experiments for doped manganites is discussed. Comment: 9 pages, 7 figures, submitted to Phys. Rev. BPhysical Review B 03/2004; · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We describe the generalization of Wilson's numerical renormalization group method to quantum impurity models with a bosonic bath, providing a general nonperturbative approach to bosonic impurity models which can access exponentially small energies and temperatures. As an application, we consider the spinboson model, describing a twolevel system coupled to a bosonic bath with powerlaw spectral density, J(omega) proportional to omega(s). We find clear evidence for a line of continuous quantum phase transitions for subOhmic bath exponents 0<s<1; the line terminates in the wellknown KosterlitzThouless transition at s=1. Contact is made with results from perturbative renormalization group, and various other applications are outlined.Physical Review Letters 10/2003; 91(17):170601. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The MottHubbard metalinsulator transition in the paramagnetic phase of the oneband Hubbard model has long been used to describe similar features in real materials like V$_2$O$_3$. Here we show that this transition is hidden inside a rather robust antiferromagnetic insulator even in the presence of comparatively strong magnetic frustration. This result raises the question of the relevance of the MottHubbard metalinsulator transition for the generic phase diagram of the oneband Hubbard model. Comment: 4 pages, 6 figuresPhysical Review Letters 08/2003; · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the unanalytical structure of metalinsulator transition (MIT) in infinite dimensions. By introducing a simple transformation into the dynamical meanfield equation of Hubbard model, a multiplevalued structure in Green's function and other thermodynamical quantities with respect to the interaction strength $U$ are found at low temperatures. A unified description of stable, metastable and unstable phases is obtained in the regime $U_{c1}(T)<U<U_{c2}(T)$, and the Maxwell construction is performed to evaluate the MIT line $U^{\ast}(T)$. We show how the firstorder MIT at $U^{\ast}(T)$ for $T>0$ evolves into secondorder one at $U_{c2}(0)$ for $T=0 $. The phase diagram near MIT is presented. Comment: 5 pages with 3 figures, text and figures revisedPhysical review. B, Condensed matter 05/2001; · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The unanalytical feature of the MottHubbard metalinsulator transition in infinite dimensions is studied by using a modified selfconsistency scheme in the framework of dynamical meanfield theory. For a specific low temperature T=0.0025W (W: half bandwidth), the ``Z''shaped or ``S''shaped curves for double occupancy D, energy per lattice site E, and the quasiparticle weight Z are obtained as functions of U. Direct observation shows that the Fermiliquid phase changes to the insulating phase through a nonFermiliquid phase at the unstable level. Based on our results, a scenario of zero temperature MIT is proposed. The phase diagram for MIT on the TD plane is also presented.Modern Physics Letters B 01/2001; 15:12491258. · 0.48 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present the fine structures of various phase separations (PS) in the doubleexchange (DE) model either with or without antiferromagnetic (AF) superexchange coupling between localized spins in the framework of dynamical meanfield theory. They constitute a complete and unified description of stable, metastable, and unstable states near PS. At meanfield level, the PS between ferromagnetic (FM) and paramagnetic phases at low temperature in the DE model and that between FM and AF phases in the DE model with AF superexchange background are found stable against nearestneighbor Coulomb repulsion. The relevance of our results to experiments and to other theoretical studies of PS is discussed.Physical Review B 09/2000; 62(14). · 3.66 Impact Factor
Publication Stats
398  Citations  
78.38  Total Impact Points  
Top Journals
Institutions

2006–2010

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


2005–2006

Karlsruhe Institute of Technology
 Institute of Theoretical Condensed Matter Physics
Karlsruhe, BadenWuerttemberg, Germany


2003–2006

Universität Augsburg
 Institute of Physics
Augsberg, Bavaria, Germany


2000–2001

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