Analytic calculation of high-order corrections to quantum phase transitions of ultracold Bose gases in bipartite superlattices

We clarify some technical issues in the present generalized effective-potential Landau theory (GEPLT) to make the GEPLT more consistent and complete. Utilizing this clarified GEPLT, we analytically study the quantum phase transitions of ultracold Bose gases in bipartite superlattices at zero temperature. The corresponding quantum phase boundaries are analytically calculated up to the third-order hopping, which are in excellent agreement with the quantum Monte Carlo (QMC) simulations.     

Quantum Monte Carlo study of hard-core bosons in Creutz ladder with zero flux

The quantum phase of hard-core bosons in Creutz ladder with zero flux is studied.For a specific regime of the parameters(t_x=t_p,t_y0),the exact ground-state is found analytically,which is a dimerized insulator with one electron bound in each rung of the ladder.For the case t_x,t_y,t_p0,the system is exactly studied using quantum Monte Carlo(QMC)method without a sign problem.It is found that the system is a Mott insulator for small t_p and a quantum phase transition to a superfluid phase is driven by increasing t_p.The critical t~c _pis determined precisely by a scaling analysis.Since it is possible that the Creutz ladder is realized experimentally,the theoretical results are interesting to the cold-atom experiments.     

Phase diagram of the three-dimensional Hubbard model at half filling

We investigate the phase diagram of the three-dimensional Hubbard model at half filling using quantum Monte Carlo (QMC) simulations. The antiferromagnetic Néel temperature is determined from the specific heat maximum in combination with finite-size scaling of the magnetic structure factor. Our results interpolate smoothly between the asymptotic solutions for weak and strong coupling, respectively, in contrast to previous QMC simulations. The location of the metal-insulator transition in the paramagnetic phase above is determined using the electronic compressibility as criterion. Received 11 April 2000 and Revised in final form 29 June 2000     

Mott-hubbard metal-insulator transition in paramagnetic V2O3: an LDA+DMFT(QMC) study

The electronic properties of paramagnetic V2O3 are investigated by the computational scheme LDA+DMFT(QMC). This approach merges the local density approximation (LDA) with dynamical mean-field theory (DMFT) and uses quantum Monte Carlo simulations (QMC) to solve the effective Anderson impurity model of DMFT. Starting with the crystal structure of metallic V2O3 and insulating (V0.962Cr0.038)2O3 we find a Mott-Hubbard transition at a Coulomb interaction U approximately 5 eV. The calculated spectrum is in very good agreement with experiment. Furthermore, the orbital occupation and the spin state S = 1 determined by us agree with recent polarization dependent x-ray-absorption experiments.     

Theory of smeared quantum phase transitions

We present an analytical strong-disorder renormalization group theory of the quantum phase transition in the dissipative random transverse-field Ising chain. For Ohmic dissipation, we solve the renormalization flow equations analytically, yielding asymptotically exact results for the low-temperature properties of the system. We find that the interplay between quantum fluctuations and Ohmic dissipation destroys the quantum critical point by smearing. We also determine the phase diagram and the behavior of observables in the vicinity of the smeared quantum phase transition.     

The effect of dilute nitrogen on nonlinear optical properties of the InGaAsN/GaAs single quantum wells

Employing the extended dynamical mean field theory (EDMFT) and the quantum Monte Carlo (QMC) method, we investigate the effect of the spatial fluctuations in the two-band Hubbard model with anisotropic bandwidth in the vicinity of the Mott metal-insulator transition. At half filling, we demonstrate that while the inclusion of the non-local spin-spin interaction amounts to enhancing the correlation and suppressing the metallic character, the orbitally selective Mott transition (OSMT) remains stable for various strengths of the non-local correlation. The same is true when the system is doped away from half filling. The OSMT phase is evidenced at low dopant concentration and the simultaneous metallic phase emerges at overdoped regime. From the analysis of the self energy, it follows that the nature of the metallic phase upon doping violates the Fermi liquid character and persists at considerably large doping. Our theory also offers a new perspective for the investigation of the non-local fluctuation in the multi-orbital system within the single-site scheme.     

Electron emission from diamondoids: a diffusion quantum Monte Carlo study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.     

双光子过程耗散耦合腔阵列中的量子相变

基于准玻色方法, 利用平均场理论解析求解了环境作用下双光子过程耦合腔阵列体系的哈密顿量, 得到了体系序参量的解析表达式, 并讨论了耗散对体系超流-Mott绝缘相变的影响. 研究结果表明: 双光子共振情况下系统重铸相干的腔间耦合率临界值为(ZJ/β)= (ZJ/β)_c'≈ 0.34;双光子相互作用过程比单光子过程具有更大的耗散率, 系统维持长程相干状态的时间更短, 而实现重铸相干的腔间耦合率的临界值更大.     

Quantum Quenches with Random Matrix Hamiltonians and Disordered Potentials

We numerically investigate statistical ensembles for the occupations of eigenstates of an isolated quantum system emerging as a result of quantum quenches. The systems investigated are sparse random matrix Hamiltonians and disordered lattices. In the former case, the quench consists of sudden switching‐on the off‐diagonal elements of the Hamiltonian. In the latter case, it is sudden switching‐on of the hopping between adjacent lattice sites. The quench‐induced ensembles are compared with the so‐called “quantum micro‐canonical” (QMC) ensemble describing quantum superpositions with fixed energy expectation values. Our main finding is that quantum quenches with sparse random matrices having one special diagonal element lead to the condensation phenomenon predicted for the QMC ensemble. Away from the QMC condensation regime, the overall agreement with the QMC predictions is only qualitative for both random matrices and disordered lattices but with some cases of a very good quantitative agreement. In the case of disordered lattices, the QMC ensemble can be used to estimate the probability of finding a particle in a localized or delocalized eigenstate.     

Abelian duality, confinement, and chiral-symmetry breaking in a SU(2) QCD-like theory

We analyze the vacuum structure of SU(2) QCD with multiple massless adjoint representation fermions formulated on a small spatial S(1) x R(3). The absence of thermal fluctuations, and the fact that quantum fluctuations favor the vacuum with unbroken center symmetry in a weakly coupled regime, renders the interesting dynamics of these theories analytically calculable. Confinement and the generation of the mass gap in the gluonic sector are shown analytically. In this regime, theory exhibits confinement without continuous chiral-symmetry breaking. However, a flavor singlet chiral condensate (which breaks a discrete chiral symmetry) persists at arbitrarily small S(1). Under certain reasonable assumptions, we show that the theory exhibits a zero temperature chiral phase transition in the absence of any change in spatial center symmetry realizations.     

Hard-Core Bosons on a Two-Dimensional Square Optical Superlattice

In this work,we theoretically study hard-core bosons on a two-dimensional square optical superlattice at T = 0.First of all,we present the mean field phase diagram of this model in terms of the chemical potential μ and the alternating potential strength △.Besides a superfluid(SF) phase at △ = 0 and a charge density wave(CDW)phase in the large △ at half filling,we demonstrate that a supersolid(SS) phase emerges in the moderate △.Then,we focus on the μ = 0,e.g.,half filling case,using large-S semi-classical spin-wave approximation to study the SS to CDW quantum phase transition.In particular,we calculate the ground-state energy and the superfluid density at the level of1/S correction.We then compare the spin-wave results with the large scale quantum Monte Carlo(QMC) simulations using the cluster stochastic series expansion(CSSE) algorithm,and find that while the spin wave method is intuitive with clear physical pictures,the quantum critical point is quite different from that of numerical results which is believed to be accurate.We suggest that as simple as it is,this model still exhibits strong quantum fluctuations near the quantum critical point beyond the power of semiclassical spin-wave approach.     

Parameter-dependent third-order optical nonlinearity in a CdSe/ZnS quantum dot quantum well in the vicinity of a gold nanoparticle

The nonlinear optical properties of the CdSe/ZnS quantum dot quantum well (QDQW) in the vicinity of a spherical metal nano-particle (MNP) have been described. The third-order nonlinear optical susceptibility induced by the transition between E₁ (inside the well) and E₂ (outside the well) has been calculated for the third-harmonic generation (THG) under the effective mass approximation and modified by the local field theory. The parameters-dependent third-order nonlinear optical susceptibility for the THG has been specifically explored and the influence of the distance between the QDQW and the MNP on the third-order susceptibility for the THG in the system has been shown and analyzed.     

Bose-Einstein Condensation of Photons and Photon Pairs

We investigate the Bose-Einstein condensation of photons and photon pairs in a two-dimension optical microcavity. We find that in the paraxial approximation, the mixed gas of photons and photon pairs is formally equivalent to a two dimension system of massive bosons with non-vanishing chemical potential, which implies the existence of two possible condensate phase. We also discuss the quantum phase transition of the system and obtain the critical point analytically. Moreover, we find that the quantum phase transition of the system can be interpreted as second harmonic generation.     

Bose-Einstein Condensation of Photons and Photon Pairs

We investigate the Bose-Einstein condensation of photons and photon pairs in a two-dimension optical microcavity. We find that in the paraxial approximation, the mixed gas of photons and photon pairs is formally equivalent to a two dimension system of massive bosons with non-vanishing chemical potential, which implies the existence of two possible condensate phase. We also discuss the quantum phase transition of the system and obtain the critical point analytically. Moreover, we find that the quantum phase transition of the system can be interpreted as second harmonic generation.     

Non-Fermi-liquid phases in the two-band Hubbard model: finite-temperature exact diagonalization study of Hund's rule coupling

The two-band Hubbard model involving subbands of different widths is investigated via finite-temperature exact diagonalization (ED) and dynamical mean field theory (DMFT). In contrast to the quantum Monte Carlo (QMC) method which at low temperatures includes only Ising-like exchange interactions to avoid sign problems, ED permits a treatment of Hund's exchange and other onsite Coulomb interactions on the same footing. The role of finite-size effects caused by the limited number of bath levels in this scheme is studied by analyzing the low-frequency behavior of the subband self-energies as a function of temperature, and by comparing with numerical renormalization group (NRG) results for a simplified effective model. For half-filled, non-hybridizing bands, the metallic and insulating phases are separated by an intermediate mixed phase with an insulating narrow and a bad-metallic wide subband. The wide band in this phase exhibits different degrees of non-Fermi-liquid behavior, depending on the treatment of exchange interactions. Whereas for complete Hund's coupling, infinite lifetime is found at the Fermi level, in the absence of spin-flip and pair-exchange, this lifetime becomes finite. Excellent agreement is obtained both with new NRG and previous QMC/DMFT calculations. These results suggest that-finite temperature ED/DMFT might be a useful scheme for realistic multi-band materials.     

The S-FFT calculation of Collins formula and its application in digital holography

We study analytically and numerically the properties of one-dimensional chain of cold ions placed in a periodic potential of optical lattice and global harmonic potential of a trap. In close similarity with the Frenkel-Kontorova model, a transition from sliding to pinned phase takes place with the increase of the optical lattice potential for the density of ions incommensurate with the lattice period. We show that at zero temperature the quantum fluctuations lead to a quantum phase transition and melting of pinned instanton glass phase at large values of dimensional Planck constant. After melting the ion chain can slide in an optical lattice. The obtained results are also relevant for a Wigner crystal placed in a periodic potential.     

Two-dimensional Rydberg gases and the quantum hard-squares model

We study a two-dimensional lattice gas of atoms that are photoexcited to Rydberg states in which they interact via the van?der?Waals interaction. We explore the regime of dominant nearest-neighbor interaction where this system is intimately connected with a quantum version of Baxter's hard-squares model. We show that the strongly correlated ground state of the Rydberg gas can be analytically described by a projected entangled pair state that constitutes the ground state of the quantum hard-squares model. This correspondence allows us to identify a phase boundary where the Rydberg gas undergoes a transition from a disordered (liquid) phase to an ordered (solid) phase.     

Calculation of photoemission spectra of the doped Mott insulator using LDA+DMFT(QMC)

The spectral properties of La_1–xSr_xTiO₃, a doped Mott insulator with strong Coulomb correlations, are calculated with the ab initio computational scheme LDA+DMFT(QMC). It starts from the non-interacting electronic band structure as calculated by the local density approximation (LDA), and introduces the missing correlations by the dynamical mean-field theory (DMFT), using numerically exact quantum Monte-Carlo (QMC) techniques to solve the resulting self-consistent multi-band single-impurity problem. The results of the LDA+DMFT(QMC) approach for the photoemission spectra of La_1–xSr_xTiO₃ are in good agreement with experiment and represent a considerable qualitative and quantitative improvement on standard LDA calculations. Received 20 May 2000 and Received in final form 27 July 2000     

Efficient quantum monte carlo energies for molecular dynamics simulations

A method is presented to treat electrons within the many-body quantum Monte Carlo (QMC) approach "on-the-fly" throughout a molecular dynamics (MD) simulation. Our approach leverages the large (10-100) ratio of the QMC electron to MD ion motion to couple the stochastic, imaginary-time electronic and real-time ionic trajectories. This continuous evolution of the QMC electrons results in highly accurate total energies for the full dynamical trajectory at a fraction of the cost of conventional, discrete sampling. We show that this can be achieved efficiently for both ground and excited states with only a modest overhead to an ab initio MD method. The accuracy of this dynamical QMC approach is demonstrated for a variety of systems, phases, and properties, including optical gaps of hot silicon quantum dots, dissociation energy of a single water molecule, and heat of vaporization of liquid water.     

Scaling of reduced fidelity susceptibility in the one-dimensional transverse-field XY model

We show that the reduced fidelity susceptibility in the family of one-dimensional XY model obeys scaling behavior in the vicinity of quantum critical points both analytically and numerically. The logarithmic divergence behavior suggests that the reduced fidelity susceptibility can act as an indicator of quantum phase transition.