arXiv subjects

N. V. Prokof'ev

Publications and source records attributed to N. V. Prokof'ev.

At least 37 records · Page 2Linked to original sources

arXiv2010

Incorporating Dynamic Mean-Field Theory into Diagrammatic Monte Carlo

The bold diagrammatic Monte Carlo (BDMC) method performs an unbiased sampling of Feynman's diagrammatic series using skeleton diagrams. For lattice models the efficiency of BDMC can be dramatically improved by incorporating dynamic mean-field theory (DMFT) solutions into renormalized propagators. From the DMFT perspective, combining it with BDCM leads to an unbiased method with well-defined accuracy. We illustrate the power of this approach by computing the single-particle propagator (and thus the density of states) in the non-perturbative regime of the Anderson localization problem, where a gain of the order of $10^4$ is achieved with respect to conventional BDMC in terms of convergence to the exact answer.

cond-mat.stat-mech↗

arXiv2010

Comment on "Direct Mapping of the Finite Temperature Phase Diagram of Strongly Correlated Quantum Models" by Q. Zhou, Y. Kato, N. Kawashima, and N. Trivedi, Phys. Rev. Lett. 103, 085701 (2009)

In their Letter, Zhou, Kato, Kawashima, and Trivedi claim that finite-temperature critical points of strongly correlated quantum models emulated by optical lattice experiments can generically be deduced from kinks in the derivative of the density profile of atoms in the trap with respect to the external potential, $κ= -dn(r)/dV(r)$. In this comment we demonstrate that the authors failed to achieve their goal: to show that under realistic experimental conditions critical densities $n_c(T,U)$ can be extracted from density profiles with controllable accuracy.

cond-mat.quant-gas↗

arXiv2010

Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipole systems

The superfluid to normal fluid transition of dipolar bosons in two dimensions is studied throughout the whole density range using path integral Monte Carlo simulations and summarized in the phase diagram. While at low densities, we find good agreement with the universal results depending only on the scattering length $a_s$, at moderate and high densities, the transition temperature is strongly affected by interactions and the elementary excitation spectrum. The results are expected to be of relevance to dipolar atomic and molecular systems and indirect excitons in quantum wells.

physics.atom-ph↗

arXiv2010

Criticality in Trapped Atomic Systems

We discuss generic limits posed by the trap in atomic systems on the accurate determination of critical parameters for second-order phase transitions, from which we deduce optimal protocols to extract them. We show that under current experimental conditions the in-situ density profiles are barely suitable for an accurate study of critical points in the strongly correlated regime. Contrary to recent claims, the proper analysis of time-of-fight images yields critical parameters accurately.

cond-mat.quant-gas↗

arXiv2009

Critical entropies for magnetic ordering in bosonic mixtures on a lattice

We perform a numeric study (worm algorithm Monte Carlo simulations) of ultracold two-component bosons in two- and three-dimensional optical lattices. At strong enough interactions and low enough temperatures the system features magnetic ordering. We compute critical temperatures and entropies for the disappearance of the Ising antiferromagnetic and the xy-ferromagnetic order and find that the largest possible entropies per particle are ~0.5kB. We also estimate (optimistically) the experimental hold times required to reach equilibrium magnetic states to be on a scale of seconds. Low critical entropies and long hold times render the experimental observations of magnetic phases challenging and call for increased control over heating sources.

cond-mat.quant-gas↗

arXiv2009

Phase diagram of the disordered Bose-Hubbard model

We establish the phase diagram of the disordered three-dimensional Bose-Hubbard model at unity filling, which has been controversial for many years. The theorem of inclusions, proven in Ref. [1], states that the Bose glass phase always intervenes between the Mott insulating and superfluid phases. Here, we note that assumptions on which the theorem is based exclude phase transitions between gapped (Mott insulator) and gapless phases (Bose glass). The apparent paradox is resolved through a unique mechanism: such transitions have to be of the Griffiths type when the vanishing of the gap at the critical point is due to a zero concentration of rare regions where extreme fluctuations of disorder mimic a {\it regular} gapless system. An exactly solvable random transverse field Ising model in one dimension is used to illustrate the point. A highly non-trivial overall shape of the phase diagram is revealed with the worm algorithm. The phase diagram features a long superfluid finger at strong disorder and on-site interaction. Moreover, bosonic superfluidity is extremely robust against disorder in a broad range of interaction parameters; it persists in random potentials nearly 50 (!) times larger than the particle half-bandwidth. Finally, we comment on the feasibility of obtaining this phase diagram in cold-atom experiments, which work with trapped systems at finite temperature.

cond-mat.dis-nn↗

arXiv2009

Superclimb of Dislocations and the Anomalous Isochoric Compressibility of Solid He4

In the experiment on superfluid transport in solid He4 [PRL {\bf 100}, 235301 (2008)], Ray and Hallock observed an {\it anomalously large isochoric compressibility}: the supersolid samples demonstrated a significant and apparently spatially uniform response of density and pressure to chemical potential, applied locally through Vycor "electrodes". We propose that the effect is due to {\it superclimb} : edge dislocations can climb because of mass transport along superfluid cores. We corroborate the scenario by {\it ab initio} simulations of an edge dislocation in solid He4 at $T=0.5K$. We argue that at low temperature the effect must be suppressed due to a crossover to the smooth dislocation.

cond-mat.other↗

arXiv2009

Suppression of the critical temperature for superfluidity near the Mott transition: validating a quantum simulator

Ultracold atomic gases in optical lattices have proven to be a controllable, tunable and clean implementation of strongly interacting quantum many-body systems. An essential prospect for such quantum simulators is their ability to map out the phase diagram of fundamental many-body model Hamiltonians. However, the results need to be validated first for representative benchmark problems via state-of-the-art numerical methods of quantum many-body theory. Here we present the first ab-initio comparison between experiments and quantum Monte Carlo simulations for strongly interacting Bose gases on a lattice for large systems (up to N = 3e5 particles). The comparison has enabled us to perform thermometry for the interacting quantum gas and to experimentally determine the finite temperature phase diagram for bosonic superfluids in an optical lattice. Our results reveal a downshift of the critical temperature as the transition to the Mott insulator is approached.

cond-mat.quant-gas↗

arXiv2009

Absence of a Direct Superfluid to Mott Insulator Transition in Disordered Bose Systems

We prove the absence of a direct quantum phase transition between a superfluid and a Mott insulator in a bosonic system with generic, bounded disorder. We also prove compressibility of the system on the superfluid--insulator critical line and in its neighborhood. These conclusions follow from a general {\it theorem of inclusions} which states that for any transition in a disordered system one can always find rare regions of the competing phase on either side of the transition line. Quantum Monte Carlo simulations for the disordered Bose-Hubbard model show an even stronger result, important for the nature of the Mott insulator to Bose glass phase transition: The critical disorder bound, $Δ_c$, corresponding to the onset of disorder-induced superfluidity, satisfies the relation $Δ_c > E_{\rm g/2}$, with $E_{\rm g/2}$ the half-width of the Mott gap in the pure system.

cond-mat.stat-mech↗

arXiv2009

Sign-Alternating Interaction Mediated by Strongly-Correlated Lattice Bosons

We reveal a generic mechanism of generating sign-alternating inter-site interactions mediated by strongly correlated lattice bosons. The ground state phase diagram of the two-component hard-core Bose-Hubbard model on a square lattice at half-integer filling factor for each component, obtained by worm algorithm Monte Carlo simulations, is strongly modified by these interactions and features the solid+superfluid phase for strong anisotropy between the hopping amplitudes. The new phase is a direct consequence of the effective nearest-neighbor repulsion between "heavy" atoms mediated by the "light" superfluid component. Due to their sign-alternating character, mediated interactions lead to a rich variety of yet to be discovered quantum phases. quantum phases.

cond-mat.other↗

arXiv2008

Expansion of a quantum gas released from an optical lattice

We analyze the interference pattern produced by ultracold atoms released from an optical lattice. Such interference patterns are commonly interpreted as the momentum distributions of the trapped quantum gas. We show that for finite time-of-flights the resulting density distribution can, however, be significantly altered, similar to a near-field diffraction regime in optics. We illustrate our findings with a simple model and realistic quantum Monte Carlo simulations for bosonic atoms, and compare the latter to experiments.

cond-mat.other↗

arXiv2008

Binding of a 3He impurity to a screw dislocation in solid 4He

Using first-principle simulations for the probability density of finding a 3He atom in the vicinity of the screw dislocation in solid 4He, we determine the binding energy to the dislocation nucleus E_B = 0.8 \pm 0.1 K and the density of localized states at larger distances. The specific heat due to 3He features a peak similar to the one observed in recent experiments, and our model can also account for the observed increase in shear modulus at low temperature. We further discuss the role of 3He in the picture of superfluid defects.

cond-mat.stat-mech↗

arXiv2008

Search for Deconfined Criticality: SU(2) Déjà Vu

Monte Carlo simulations of the SU(2)-symmetric deconfined critical point action reveal strong violations of scale invariance for the deconfinement transition. We find compelling evidence that the generic runaway renormalization flow of the gauge coupling is to a weak first order transition, similar to the case of U(1)$\times$U(1) symmetry. Our results imply that recent numeric studies of the Nèel antiferromagnet to valence bond solid quantum phase transition in SU(2)-symmetric models were not accurate enough in determining the nature of the transition.

cond-mat.stat-mech↗

arXiv2008

Topological multicritical point in the Toric Code and 3D gauge Higgs Models

We report a new type of multicritical point that arises from competition between the Higgs and confinement transitions in a Z_2 gauge system. The phase diagram of the 3d gauge Higgs model has been obtained by Monte-Carlo simulation on large (up to 60^3) lattices. We find the transition lines continue as 2nd-order until merging into a 1st-order line. These findings pose the question of an effective field theory for a multicritical point involving noncommuting order parameters. A similar phase diagram is predicted for the 2-dimensional quantum toric code model with two external fields, h_z and h_x; this problem can be mapped onto an anisotropic 3D gauge Higgs model.

cond-mat.stat-mech↗

arXiv2008

Phase diagram of anisotropic boson t-J model

We have studied by Quantum Monte Carlo simulations the low temperature phase diagram of a mixture of isotopic, hard core bosons, described by the t-Jz-Jperp model, with Jperp=a Jz. Coexistence of superfluid hole-rich and insulating, antiferromagnetically ordered hole-free phases is observed at sufficiently low hole density, for any a < 1. A two-component checkerboard supersolid phase is not observed. The experimental relevance and possible broader implications of these findings are discussed.

cond-mat.stat-mech↗

arXiv2007

Superfluid-Insulator and Roughening Transitions in Domain Walls

We have performed quantum Monte Carlo simulations to investigate the superfluid behavior of one- and two-dimensional interfaces separating checkerboard solid domains. The system is described by the hard-core Bose-Hubbard Hamiltonian with nearest-neighbor interaction. In accordance with Ref.1, we find that (i) the interface remains superfluid in a wide range of interaction strength before it undergoes a superfluid-insulator transition; (ii) in one dimension, the transition is of the Kosterlitz-Thouless type and is accompanied by the roughening transition, driven by proliferation of charge 1/2 quasiparticles; (iii) in two dimensions, the transition belongs to the 3D U(1) universality class and the interface remains smooth. Similar phenomena are expected for domain walls in quantum antiferromagnets.

cond-mat.other↗

arXiv2007

Superfluidity of Grain Boundaries in Solid Helium-4

By large-scale quantum Monte Carlo simulations we show that grain boundaries in Helium-4 crystals are generically superfluid at low temperature, with a transition temperature of the order of ~0.5K at the melting pressure; insulating grain boundaries are found only for special orientations of the grains. We also find that close vicinity to the melting line is not a necessary condition for superfluid grain boundaries, and a grain boundary in direct contact with the superfluid liquid at the melting curve is found to be mechanically stable and the grain boundary superfluidity observed by Sasaki et al. [Science 313, 1098 (2006)] is not just a crack filled with superfluid.

cond-mat.stat-mech↗

arXiv2007

Luttinger Liquid in the Core of Screw Dislocation in Helium-4

On the basis of first-principle Monte Carlo simulations we find that the screw dislocation along the hexagonal axis of an hcp He4 crystal features a superfluid core. This is the first example of a regular quasi-one-dimensional supersolid, and one of the cleanest cases of a regular Luttinger-liquid system. In contrast, the same type of screw dislocation in solid Hydrogen is insulating.

cond-mat.other↗