Open Access. Powered by Scholars. Published by Universities.®

Physics Commons

Open Access. Powered by Scholars. Published by Universities.®

Selected Works

Nikolai Prokof'ev

Articles 1 - 30 of 44

Full-Text Articles in Physics

Detecting Supercounterfluidity By Ramsey Spectroscopy, A Kuklov, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Detecting Supercounterfluidity By Ramsey Spectroscopy, A Kuklov, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

A two-component system of ultracold atoms in an optical lattice at integer total filling factor and strong enough onsite repulsion can form a supercounterfluid (SCF) phase, which can be viewed as the Bose-Einstein condensate (BEC) of pairs formed by particles of one sort and holes of another sort. In this quasimolecular BEC, no single-component BEC exists, and the net atomic flow is prohibited. We show that, in the case of the interconvertible species (like hyperfine states of Rb), the corresponding order parameter can be detected by spatially selective Ramsey spectroscopy. The method can be used, in particular, for revealing a ...


Worm Algorithm And Diagrammatic Monte Carlo: A New Approach To Continuous-Space Path Integral Monte Carlo Simulations, M Boninsegni, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Worm Algorithm And Diagrammatic Monte Carlo: A New Approach To Continuous-Space Path Integral Monte Carlo Simulations, M Boninsegni, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

A detailed description is provided of a new worm algorithm, enabling the accurate computation of thermodynamic properties of quantum many-body systems in continuous space, at finite temperature. The algorithm is formulated within the general path integral Monte Carlo (PIMC) scheme, but also allows one to perform quantum simulations in the grand canonical ensemble, as well as to compute off-diagonal imaginary-time correlation functions, such as the Matsubara Green function, simultaneously with diagonal observables. Another important innovation consists of the expansion of the attractive part of the pairwise potential energy into elementary (diagrammatic) contributions, which are then statistically sampled. This affords a ...


Superfluid Interfaces In Quantum Solids, E Bourovski, E. Kozik, A Kuklov, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Superfluid Interfaces In Quantum Solids, E Bourovski, E. Kozik, A Kuklov, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

One scenario for the nonclassical moment of inertia of solid 4He discovered by Kim and Chan [Nature (London) 427, 225 (2004)] is the superfluidity of microcrystallite interfaces. On the basis of the most simple model of a quantum crystal—the checkerboard lattice solid—we show that the superfluidity of interfaces between solid domains can exist in a wide range of parameters. At strong enough interparticle interaction, a superfluid interface becomes an insulator via a quantum phase transition. Under the conditions of particle-hole symmetry, the transition is of the standard U(1) universality class in 3D, while in 2D the onset ...


Decoherence And Quantum Walks: Anomalous Diffusion And Ballistic Tails, Nikolai Prokof'ev, P.C.E Stamp Mar 2011

Decoherence And Quantum Walks: Anomalous Diffusion And Ballistic Tails, Nikolai Prokof'ev, P.C.E Stamp

Nikolai Prokof'ev

The common perception is that strong coupling to the environment will always render the evolution of the system density matrix quasiclassical (in fact, diffusive) in the long time limit. We present here a counterexample, in which a particle makes quantum transitions between the sites of a d-dimensional hypercubic lattice while strongly coupled to a bath of two-level systems that “record” the transitions. The long-time evolution of an initial wave packet is found to be most unusual: the mean square displacement of the particle density matrix shows long-range ballistic behavior, with ⟨n2⟩∼t2, but simultaneously a kind of weakly localized behavior ...


Supercurrent Stability In A Quasi-One-Dimensional Weakly Interacting Bose Gas, Y Kagan, Nikolai Prokof'ev, B Svistunov Mar 2011

Supercurrent Stability In A Quasi-One-Dimensional Weakly Interacting Bose Gas, Y Kagan, Nikolai Prokof'ev, B Svistunov

Nikolai Prokof'ev

We discuss the possibility of observing superfluid phenomena in a quasi-one-dimensional weakly interacting Bose gas at finite temperature. The weakness of interaction in combination with generic properties of one-dimensional liquids can result in a situation where the relaxation time of the supercurrent flow is much longer than the system lifetime, and the behavior of the system is indistinguishable from that of a genuine superfluid.


Superfluid-Insulator Transition In Commensurate Disordered Bosonic Systems: Large-Scale Worm Algorithm Simulations, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Superfluid-Insulator Transition In Commensurate Disordered Bosonic Systems: Large-Scale Worm Algorithm Simulations, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

We report results of large-scale Monte Carlo simulations of superfluid-insulator transitions in disordered commensurate 2D bosonic systems. In the off-diagonal disorder case, we find that the transition is to a gapless incompressible insulator, and its dynamical critical exponent is z=1.5(2). In the diagonal-disorder case, we prove the conjecture that rare statistical fluctuations are inseparable from critical fluctuations on the largest scales and ultimately result in crossover to the generic universality class (apparently with z=2). However, even at strong disorder, the universal behavior sets in only at very large space-time distances. This explains why previous studies of ...


What Makes A Crystal Supersolid?, Nikolai Prokof'ev Mar 2011

What Makes A Crystal Supersolid?, Nikolai Prokof'ev

Nikolai Prokof'ev

For 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 ...


Search For Deconfined Criticality: Su(2) D´Ej`A Vu, B Kuklov, M Matsumoto, Nikolai Prokof'ev, Boris Svistunov, M Troyer Mar 2011

Search For Deconfined Criticality: Su(2) D´Ej`A Vu, B Kuklov, M Matsumoto, Nikolai Prokof'ev, Boris Svistunov, M Troyer

Nikolai Prokof'ev

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)×U(1) symmetry. Our results imply that recent numeric studies of the N`eel antiferromagnet to valence bond solid quantum phase transition in SU(2)-symmetric models were not accurate enough in determining the nature of the transition.


Superfluid-Insulator And Roughening Transitions In Domain Walls, S Söyler, Capogrosso-Sansone, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Superfluid-Insulator And Roughening Transitions In Domain Walls, S Söyler, Capogrosso-Sansone, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

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 Burovski et al. [Phys. Rev. Lett. 94, 165301 (2005)] 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 ...


Theory Of The Spin Bath, Nikolai Prokof'ev, P.C.E Stamp Mar 2011

Theory Of The Spin Bath, Nikolai Prokof'ev, P.C.E Stamp

Nikolai Prokof'ev

The 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 ...


Reply To “Comment On ‘Hole Digging In Ensembles Of Tunneling Molecular Magnets’ ”, I Tupitsyn, P Stamp, Nikolai Prokof'ev Mar 2011

Reply To “Comment On ‘Hole Digging In Ensembles Of Tunneling Molecular Magnets’ ”, I Tupitsyn, P Stamp, Nikolai Prokof'ev

Nikolai Prokof'ev

Our work has argued for a particular scaling form governing the distribution M(ξ,t) of magnetization over bias ξ, for a system of dipolar-interacting molecular spins. This form, which was found in Monte Carlo (MC) simulations, leads inevitably to a short-time form ∼t1∕2 for the magnetization relaxation in the system. The authors of the Comment argue that the magnetization should decay rather as ∼tp, with the exponent p depending on the lattice type—and they argue this form is valid up to infinite times. They also claim that our conclusion is based on an assumed exponential dependence of ...


Exact, Complete, And Universal Continuous-Time Worldline Monte Carlo Approach To The Statistics Of Discrete Quantum Systems, Nikolai Prokof'ev, Boris Svistunov, Tupitsyn Mar 2011

Exact, Complete, And Universal Continuous-Time Worldline Monte Carlo Approach To The Statistics Of Discrete Quantum Systems, Nikolai Prokof'ev, Boris Svistunov, Tupitsyn

Nikolai Prokof'ev

We show how the worldline quantum Monte Carlo procedure, which usually relies on an artificial time discretization, can be formulated directly in continuous time, rendering the scheme exact. For an arbitrary system with discrete Hilbert space, none of the configuration update procedures contain small parameters. We find that the most effective update strategy involves the motion of worldline discontinuities (both in space and time), i.e., the evaluation of the Green’s function. Being based on local updates only, our method nevertheless allows one to work with the grand canonical ensemble and nonzero winding numbers, and to calculate any dynamical ...


Critical Point Of A Weakly Interacting Two-Dimensional Bose Gas, N Prokof'ev, O Ruebenacker, B Svistunov Mar 2011

Critical Point Of A Weakly Interacting Two-Dimensional Bose Gas, N Prokof'ev, O Ruebenacker, B Svistunov

Nikolai Prokof'ev

We study the Berezinskii-Kosterlitz-Thouless transition in a weakly interacting 2D quantum Bose gas using the concept of universality and numerical simulations of the classical |ψ|4 model on a lattice. The critical density and chemical potential are given by relations nc = (mT/2πħ2)ln(ξħ2/mU) and μc = (mTU/πħ2)ln(ξμħ2/mU), where T is the temperature, m is the mass, and U is the effective interaction. The dimensionless constant ξ = 380±3 is very large and thus any quantitative analysis of the experimental data crucially depends on its value. For ξμ our result is ξμ = 13.2±0 ...


Superfluidity Of Grain Boundaries In Solid 4he, L Pollet, M Boninsegni, A Kuklov, Nikolai Prokof'ev, Boris Svistunov, M Troyer Mar 2011

Superfluidity Of Grain Boundaries In Solid 4he, L Pollet, M Boninsegni, A Kuklov, Nikolai Prokof'ev, Boris Svistunov, M Troyer

Nikolai Prokof'ev

By large-scale quantum Monte Carlo simulations we show that grain boundaries in 4He crystals are generically superfluid at low temperature, with a transition temperature of the order of ∼0.5  K at the melting pressure; nonsuperfluid 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 ...


On-Site Number Statistics Of Ultracold Lattice Bosons, B Capogrosso-Sansone, E Kozik, Nikolai Prokof'ev, Boris Svistunov Mar 2011

On-Site Number Statistics Of Ultracold Lattice Bosons, B Capogrosso-Sansone, E Kozik, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

We 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 ...


Two-Dimensional Weakly Interacting Bose Gas In The Fluctuation Region, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Two-Dimensional Weakly Interacting Bose Gas In The Fluctuation Region, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

We study the crossover between the mean-field and critical behavior of the two-dimensional Bose gas throughout the fluctuation region of the Berezinskii-Kosterlitz-Thouless phase transition point. We argue that this crossover is described by universal (for all weakly interacting |ψ|4 models) relations between thermodynamic parameters of the system, including superfluid and quasicondensate densities. We establish these relations with high-precision Monte Carlo simulations of the classical |ψ|4 model on a lattice, and check their asymptotic forms against analytic expressions derived on the basis of the mean-field theory.


Absence Of A Direct Superfluid To Mott Insulator Transition In Disordered Bose Systems, L Pollet, N Prokof'ev, B Svistunov, Troyer Mar 2011

Absence Of A Direct Superfluid To Mott Insulator Transition In Disordered Bose Systems, L Pollet, N Prokof'ev, B Svistunov, Troyer

Nikolai Prokof'ev

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 the compressibility of the system on the superfluid–insulator critical line and in its neighborhood. These conclusions follow from a general 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 ...


Fate Of Vacancy-Induced Supersolidity In 4he, M Boninsegni, A Kuklov, L Pollet, Nikolai Prokof'ev, Boris Svistunov, M Troyer Mar 2011

Fate Of Vacancy-Induced Supersolidity In 4he, M Boninsegni, A Kuklov, L Pollet, Nikolai Prokof'ev, Boris Svistunov, M Troyer

Nikolai Prokof'ev

The supersolid state of matter, exhibiting nondissipative flow in solids, has been elusive for 35 years. The recent discovery of a nonclassical moment of inertia in solid 4He by Kim and Chan provided the first experimental evidence, although the interpretation in terms of supersolidity of the ideal crystal phase remains a subject to debate. Using quantum Monte Carlo methods we investigate the long-standing question of vacancy-induced superflow and find that vacancies in a 4He crystal phase separate instead of forming a supersolid. On the other hand, nonequilibrium vacancies relaxing on defects of polycrystalline samples could provide an explanation for the ...


Superfluid Transition In A Bose Gas With Correlated Disorder, S Pilati, S Giorgini, Nikolai Prokof'ev Mar 2011

Superfluid Transition In A Bose Gas With Correlated Disorder, S Pilati, S Giorgini, Nikolai Prokof'ev

Nikolai Prokof'ev

The superfluid transition of a three-dimensional gas of hard-sphere bosons in a disordered medium is studied using quantum Monte Carlo methods. Simulations are performed in continuous space both in the canonical and in the grand-canonical ensemble. At fixed density we calculate the shift of the transition temperature as a function of the disorder strength, while at fixed temperature we determine both the critical chemical potential and the critical density separating normal and superfluid phases. In the regime of strong disorder the normal phase extends up to large values of the degeneracy parameter, and the critical chemical potential exhibits a linear ...


The Beliaev Technique For A Weakly Interacting Bose Gas, B Capogrosso-Sansone, S Giorgini, S Pilati, L. Pollet, N Prokof'ev, B Svistunov, M Troyer Mar 2011

The Beliaev Technique For A Weakly Interacting Bose Gas, B Capogrosso-Sansone, S Giorgini, S Pilati, L. Pollet, N Prokof'ev, B Svistunov, M Troyer

Nikolai Prokof'ev

Aiming at simplicity of explicit equations and, at the same time, controllable accuracy of the theory, we present our results for all the thermodynamic quantities and correlation functions for a weakly interacting Bose gas at short-to-intermediate distances obtained within an improved version of Beliaev's diagrammatic technique. With a controllably small (but essentially finite) Bogoliubov's symmetry-breaking term, Beliaev's diagrammatic technique becomes regular in the infrared limit. Up to higher-order terms (for which we present parametric order-of-magnitude estimates), the partition function and entropy of the system formally correspond to those of a non-interacting bosonic (pseudo-)Hamiltonian with a temperature-dependent ...


Hole Digging In Ensembles Of Tunneling Molecular Magnets, I Tupitsyn, P Stamp, N Prokof'ev Mar 2011

Hole Digging In Ensembles Of Tunneling Molecular Magnets, I Tupitsyn, P Stamp, N Prokof'ev

Nikolai Prokof'ev

The nuclear spin-mediated quantum relaxation of ensembles of tunneling magnetic molecules causes a “hole” to appear in the distribution of internal fields in the system. The form of this hole and its time evolution, are studied using Monte Carlo simulations. It is shown that the line shape of the tunneling hole in a partially depolarized sample must have a Lorentzian line shape. The short-time half-width ξo in Fe8 crystals should be ∼E0, the half-width of the nuclear spin multiplet, but this result is not generally true. The Lorentzian hole line shape and the short-time √t relaxation in weakly polarized samples ...


Truncated-Determinant Diagrammatic Monte Carlo For Fermions With Contact Interaction, E Bourovski, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Truncated-Determinant Diagrammatic Monte Carlo For Fermions With Contact Interaction, E Bourovski, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

For some models of interacting fermions the known solution to the notorious sign problem in Monte Carlo (MC) simulations is to work with macroscopic fermionic determinants; the price, however, is a macroscopic scaling of the numerical effort spent on elementary local updates. We find that the ratio of two macroscopic determinants can be found with any desired accuracy by considering truncated (local in space and time) matices. In this respect, MC for interacting fermionic systems becomes similar to that for the sign-problem-free bosonic systems with system-size independent update cost. We demonstrate the utility of the truncated-determinant method by simulating the ...


Sign-Alternating Interaction Mediated By Strongly Correlated Lattice Bosons, S Söyler, B Capogrosso-Sansone, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Sign-Alternating Interaction Mediated By Strongly Correlated Lattice Bosons, S Söyler, B Capogrosso-Sansone, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

We reveal a generic mechanism of generating sign-alternating intersite 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 (SF) phase for strong asymmetry between the hopping amplitudes. The new phase is a direct consequence of the effective nearest-neighbor repulsion between 'heavy' atoms mediated by the 'light' SF component. Due to their sign-alternating character, mediated interactions lead to a rich variety of yet ...


Comprehensive Study Of Fröhlich Polaron, A Mishchenko, N Prokof'ev, B Svistunov, A Sakamoto Mar 2011

Comprehensive Study Of Fröhlich Polaron, A Mishchenko, N Prokof'ev, B Svistunov, A Sakamoto

Nikolai Prokof'ev

A 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.


Worm Algorithms For Classical Statistical Models, Nikolai Prokof'ev, Boris Svistunov Mar 2011

Worm Algorithms For Classical Statistical Models, Nikolai Prokof'ev, Boris Svistunov

Nikolai Prokof'ev

We show that high-temperature expansions provide a basis for the novel approach to efficient Monte Carlo simulations. “Worm” algorithms utilize the idea of updating closed-path configurations (produced by high-temperature expansions) through the motion of end points of a disconnected path. An amazing result is that local, Metropolis-type schemes using this approach appear to have dynamical critical exponents close to zero (i.e., their efficiency is comparable to the best cluster methods) as proved by finite-size scaling of the autocorrelation time for various universality classes.


Sharp Transition For Single Polarons In The One-Dimensional Su-Schrieffer-Heeger Model, D Marchand, G De Filippis, V Cataudella, M Berciu, N Nagaosa, Nikolai Prokof'ev, A Mishchenko, P Stamp Mar 2011

Sharp Transition For Single Polarons In The One-Dimensional Su-Schrieffer-Heeger Model, D Marchand, G De Filippis, V Cataudella, M Berciu, N Nagaosa, Nikolai Prokof'ev, A Mishchenko, P Stamp

Nikolai Prokof'ev

We 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 ...


Effective Hamiltonian In The Problem Of A "Central Spin" Coupled To A Spin Environment, I Tupitsyn, Nikolai Prokof'ev, P Stamp Mar 2011

Effective Hamiltonian In The Problem Of A "Central Spin" Coupled To A Spin Environment, I Tupitsyn, Nikolai Prokof'ev, P Stamp

Nikolai Prokof'ev

We consider here the problem of a "giant spin", with spin quantum number S≫1, interacting with a set of microscopic spins. Interactions between the microscopic spins are ignored. This model describes the low-energy properties of magnetic grains or magnetic macromolecules (ferromagnetically or antiferromagnetically ordered) interacting with a surrounding spin environment, such as nuclear spins. Our aim is to give a general method for truncating the model to another one, valid at low energies, in which a two-level system interacts with the environmental spins, and higher energy terms are absorbed into a new set of couplings. This is done using ...


Supersolid Phase Of Hard-Core Bosons On A Triangular Lattice, M Boninsegni, Nikolai Prokof'ev Mar 2011

Supersolid Phase Of Hard-Core Bosons On A Triangular Lattice, M Boninsegni, Nikolai Prokof'ev

Nikolai Prokof'ev

We study properties of the supersolid phase observed for hard-core bosons on the triangular lattice near half-integer filling factor, and the phase diagram of the system at finite temperature. We find that the solid order is always of the (2m,-m′,-m′) with m changing discontinuously from positive to negative values at half filling, in contrast with phases observed for Ising spins in a transverse magnetic field. At finite temperature we find two intersecting second-order transition lines: one in the 3-state Potts universality class and the other of the Kosterlitz-Thouless type.


Local Stress And Superfluid Properties Of Solid 4he, L Pollet, M Boninsegni, A Kuklov, Nikolai Prokof'ev, Boris Svistunov, M Troyer Mar 2011

Local Stress And Superfluid Properties Of Solid 4he, L Pollet, M Boninsegni, A Kuklov, Nikolai Prokof'ev, Boris Svistunov, M Troyer

Nikolai Prokof'ev

We provide a semiquantitative tool, derived from first-principles simulations, for answering the question of whether certain types of defects in solid 4He support mass superflow. Although ideal crystals of 4He are not supersolid, the gap for vacancy creation closes when applying a moderate stress. While a homogeneous system becomes unstable at this point, the stressed core of crystalline defects (dislocations and grain boundaries) can turn superfluid.


The Fermi–Hubbard Model At Unitarity, F Bourovski, Nikolai Prokof'ev, Boris Svistunov, M Troyer Mar 2011

The Fermi–Hubbard Model At Unitarity, F Bourovski, Nikolai Prokof'ev, Boris Svistunov, M Troyer

Nikolai Prokof'ev

We simulate the dilute attractive Fermi–Hubbard model in the unitarity regime using a diagrammatic determinant Monte Carlo (MC) algorithm with worm-type updates. We obtain the dependence of the critical temperature on the filling factor ν and, by extrapolating to ν → 0, determine the universal critical temperature of the continuum unitary Fermi gas in units of Fermi energy: Tc/εF = 0.152(7). We also determine the thermodynamic functions and show how the MC results can be used for accurate thermometry of a trapped unitary gas.