Prokof'ev, Nikolai
Loading...
Email Address
Birth Date
Research Projects
Organizational Units
Job Title
Professor, Physics Department
Last Name
Prokof'ev
First Name
Nikolai
Discipline
Physics
Expertise
Introduction
Name
84 results
Search Results
Now showing 1 - 10 of 84
Publication Open Access Comment on “Phase Diagram of a Disordered Boson Hubbard Model in Two Dimensions”(2003-01) Prokof’ev, N; Svistunov, BIn a recent Letter [1] (see also [2]) the authors presented numerical evidence supporting an idea of a direct transition between the superfluid (SF) and Mott insulating (MI) phases in the disordered Bosonic system, and even studied non-trivial properties of the multicritical line where SF, MI and the Bose Glass (BG) phases meet. The results were obtained from Monte Carlo simulations of the (2+1)-dimensional classical loop-current model [3] with the lattice action S = 1 2K Þ E ~ J=0 XrƒÑ ~ J2(r, ƒÑ) . 2(ƒÊ + v(r)) ~ JƒÑ (r, ƒÑ) . (1) where r, ƒÑ are spatial and imaginary time coordinates, and ~ J(r, ƒÑ) are integer current vectors with zero divergence. The spatial disorder potential v(r) is uniformly distributed on the interval (.,).Publication Open Access Comment on “Hausdorff Dimension of Critical Fluctuations in Abelian Gauge Theories”(2006-01) Prokof'ev, N; Svistunov, BA Comment on the Letter by J. Hove, S. Mo, and A. Sudbø Phys. Rev. Lett. 85, 2368 (2000). The authors of the Letter offer a Reply.Publication Open Access Polaron Mobility in the "Beyond Quasiparticles" Regime(2019-01) Mishchenko, Andrey S.; Pollet, Lode; Prokof'ev, Nikolay V.; Kumar, Abhishek; Maslov, Dmitri L.; Nagaosa, NaotoIn a number of physical situations, from polarons to Dirac liquids and to non-Fermi liquids, one encounters the “beyond quasiparticles” regime, in which the inelastic scattering rate exceeds the thermal energy of quasiparticles. Transport in this regime cannot be described by the kinetic equation. We employ the diagrammatic Monte Carlo method to study the mobility of a Fröhlich polaron in this regime and discover a number of nonperturbative effects: a strong violation of the Mott-Ioffe-Regel criterion at intermediate and strong couplings, a mobility minimum at T∼Ω in the strong-coupling limit (Ω is the optical mode frequency), a substantial delay in the onset of an exponential dependence of the mobility for T<Ω at intermediate coupling, and complete smearing of the Drude peak at strong coupling. These effects should be taken into account when interpreting mobility data in materials with strong electron-phonon coupling.Publication Open Access Theory of the spin bath(2000-01) Prokof'ev, Nikolai; Stamp, P.C.EThe quantum dynamics of mesoscopic or macroscopic systems is always complicated by their coupling to many `environmental' modes. At low T these environmental effects are dominated by localized modes, such as nuclear and paramagnetic spins, and defects (which also dominate the entropy and specific heat). This environment, at low energies, maps onto a `spin bath' model. This contrasts with `oscillator bath' models (originated by Feynman and Vernon) which describe delocalized environmental modes such as electrons, phonons, photons, magnons, etc. The couplings to N spin bath modes are independent of N (rather than the ~O(1/(N )1/2 ) dependence typical of oscillator baths), and often strong. One cannot in general map a spin bath to an oscillator bath (or vice versa); they constitute distinct `universality classes' of quantum environment. We show how the mapping to spin bath models is made, and then discuss several examples in detail, including moving particles, magnetic solitons, nanomagnets, and SQUIDs, coupled to nuclear and paramagnetic spin environments. We then focus on the `central spin' model, which couples a central two-level system to a background spin bath. It is the spin bath analogue of the famous `spin-boson' oscillator model, and describes, e.g., the tunnelling dynamics of nanoscopic and mesoscopic magnets and superconductors. We show how to average over (or `integrate out') spin bath modes, using an operator instanton technique, to find the central spin dynamics. The formal manouevres involve four separate averages - each average corresponds physically to a different `decoherence' mechanism acting on the central spin dynamics. Each environmental spin has its own topological `spin phase', which by interacting with the phase of the central system, decoheres it - this can happen even without dissipation. We give analytic results for the central spin correlation functions, under various conditions. We then describe the application of this theory to magnetic and superconducting systems. Particular attention is given to recent work on tunnelling magnetic macromolecules, where the role of the nuclear spin bath in controlling the tunnelling is very clear; we also discuss other magnetic systems in the quantum regime, and the influence of nuclear and paramagnetic spins on flux dynamics in SQUIDs. Finally, we discuss decoherence mechanisms and coherence experiments in superconductors and magnets. We show that a spin bath environment causes decoherence even in the Trightarrow 0 limit. Control of this decoherence will be essential in the effort to construct `qubits' for quantum computers.Publication Open Access On-site number statistics of ultracold lattice bosons(2007-01) Capogrosso-Sansone, B; Kozik, E; Prokof'ev, Nikolai; Svistunov, BorisWe study on-site occupation number fluctuations in a system of interacting bosons in an optical lattice. The ground-state distribution is obtained analytically in the limiting cases of strong and weak interaction, and by means of exact Monte Carlo simulations in the strongly correlated regime. As the interaction is increased, the distribution evolves from Poissonian in the noninteracting gas to a sharply peaked distribution in the Mott-insulator (MI) regime. In the special case of large occupation numbers, we demonstrate analytically and check numerically that there exists a wide interval of interaction strength, in which the on-site number fluctuations remain Gaussian and are gradually squeezed until they are of order unity near the superfluid (SF)-MI transition. Recently, the on-site number statistics were studied experimentally in a wide range of lattice potential depths [Phys. Rev. Lett. 96, 090401 (2006)]. In our simulations, we are able to directly reproduce experimental conditions using temperature as the only free parameter. Pronounced temperature dependence suggests that measurements of on-site atom number fluctuations can be employed as a reliable method of thermometry in both SF and MI regimes.Publication Open Access What makes a crystal supersolid?(2007-01) Prokof'ev, NikolaiFor nearly half a century the supersolid phase of matter has remained mysterious, not only eluding experimental observation, but also generating a great deal of controversy among theorists. The recent discovery of what is interpreted as a non-classical moment of inertia at low temperature in solid 4He [E. Kim and M.H.W. Chan, Nature 427 225 (2004a); E. Kim and M.H.W. Chan, Science 305 1941 (2004b); E. Kim and M.H.W. Chan, Phys. Rev. Lett. 97 115302 (2006); A.C. Clark and M.H.W. Chan, J. Low Temp. Phys. 138 853 (2005)] has elicited much excitement as a possible first observation of a supersolid phase. In the two years following the discovery, however, more puzzles than answers have been provided to the fundamental issue of whether the supersolid phase exists, in helium or any other naturally occurring condensed matter system. Presently, there is no established theoretical framework to understand the body of experimental data on 4He. Different microscopic mechanisms that have been suggested to underlie superfluidity in a perfect quantum crystal do not seem viable for 4He, for which a wealth of experimental and theoretical evidence points to an insulating crystalline ground state. This perspective addresses some of the outstanding problems with the interpretation of recent experimental observations of the apparent superfluid response in 4He (seen now by several groups, e.g. A.S. Rittner and J.D. Reppy 2006; M. Kondo, S. Takada, Y. Shibayama and K. Shirahama, Proceedings of QFS2006, Kyoto, Submitted to J. Low Temp. Phys.; A. Penzyev, Y. Yasuta and M. Kubota, Proceedings of QFS2006, Kyoto, Submitted to J. Low Temp. Phys., cond-mat/0702632.) and discusses various scenarios alternative to the homogeneous supersolid phase, such as superfluidity induced by extended defects of the crystalline structure, including grain boundaries, dislocations and anisotropic stresses. Can a metastable superfluid 'glassy' phase exist, and can it be relevant to some of the experimental observations? One of the most interesting and unsolved fundamental questions is what interatomic potentials, given the freedom to design one, can support an ideal supersolid phase in continuous space, and can they be found in Nature.Publication Open Access Sharp Transition for Single Polarons in the One-Dimensional Su-Schrieffer-Heeger Model(2010-01) Marchand, D; De Filippis, G; Cataudella, V; Berciu, M; Nagaosa, N; Prokof'ev, Nikolai; Mishchenko, A; Stamp, PWe study a single polaron in the Su-Schrieffer-Heeger (SSH) model using four different techniques (three numerical and one analytical). Polarons show a smooth crossover from weak to strong coupling, as a function of the electron-phonon coupling strength λ, in all models where this coupling depends only on phonon momentum q. In the SSH model the coupling also depends on the electron momentum k; we find it has a sharp transition, at a critical coupling strength λc, between states with zero and nonzero momentum of the ground state. All other properties of the polaron are also singular at λ=λc. This result is representative of all polarons with coupling depending on k and q, and will have important experimental consequences (e.g., in angle-resolved photoemission spectroscopy and conductivity experiments).Publication Open Access Spin bath-mediated decoherence in superconductors(2000-01) Prokof'ev, Nikolai; Stamp, P.C.EWe consider a SQUID tunneling between 2 nearly degenerate flux states. Decoherence caused by paramagnetic and nuclear spins in the low-T limit is shown to be much stronger than that from electronic excitations. The decoherence time ô is determined by the linewidth Eo of spin bath states, which can be reduced by a correct choice of ring geometry and isotopic purification. Eo can be measured in either field sweep or microwave absorption experiments, allowing both a test of the theory and design control.Publication Open Access Diagrammatic Quantum Monte Carlo for Two-Body Problems: Applied to Excitons(2001-01) Burovski, E; Mishchenko, A; Prokof'ev, Nikolai; Svistunov, BWe present a novel method for the precise numeric solution of the irreducible two-body problem and apply it to excitons in solids. The approach is based on the Monte Carlo simulation of the two-body Green function specified by Feynman’s diagrammatic expansion. Our method does not rely on the specific form of the electron and hole dispersion laws and is valid for any attractive electron-hole potential. We establish limits of validity of the Wannier (large radius) and Frenkel (small radius) approximations, present accurate data for the intermediate radius excitons, and give evidence for the charge transfer nature of the monopolar exciton in mixed valence materials.Publication Open Access Comprehensive study of Fröhlich polaron(2000-01) Mishchenko, A; Prokof'ev, N; Svistunov, B; Sakamoto, AA study of the Fröhlich polaron model is performed on the basis of diagrammatic quantum Monte Carlo technique which is enhanced by novel method of spectral analysis of the polaron Green function. We make available for the first time precise data for the effective mass, including the region of intermediate and strong couplings, and analyze the structure of the polaron cloud. A non-trivial structure of the spectral density is observed: at high enough couplings the spectral continuum features pronounced peaks that we attribute to unstable excited states of the polaron.