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Piotr Magierski

Publications and source records attributed to Piotr Magierski.

At least 55 records · Page 3Linked to original sources
arXiv2012

The Equation of State of the Unitary Fermi Gas: An Update on Lattice Calculations

The thermodynamic properties of the unitary Fermi gas (UFG) have recently been measured to unprecedented accuracy at the MIT. In particular, these measurements provide an improved understanding of the regime below T/eF ~ 0.20, where a transition into a superfluid phase occurs. In light of this development, we present an overview of state-of-the-art auxiliary field quantum Monte Carlo (AFQMC) results for the UFG at finite temperature and compare them with the MIT data for the energy, chemical potential, and density. These AFQMC results have been obtained using methods based on the hybrid Monte Carlo (HMC) algorithm, which was first introduced within the context of lattice QCD.

cond-mat.quant-gas↗
arXiv2012

The Unitary Fermi Gas: From Monte Carlo to Density Functionals

In this chapter, we describe three related studies of the universal physics of two-component unitary Fermi gases with resonant short-ranged interactions. First we discuss an ab initio auxiliary field quantum Monte Carlo technique for calculating thermodynamic properties of the unitary gas from first principles. We then describe in detail a Density Functional Theory (DFT) fit to these thermodynamic properties: the Superfluid Local Density Approximation (SLDA) and its Asymmetric (ASLDA) generalization. We present several applications, including vortex structure, trapped systems, and a supersolid Larkin-Ovchinnikov (FFLO/LOFF) state. Finally, we discuss the time-dependent extension to the density functional (TDDFT) which can describe quantum dynamics in these systems, including non-adiabatic evolution, superfluid to normal transitions and other modes not accessible in traditional frameworks such as a Landau-Ginzburg, Gross-Pitaevskii, or quantum hydrodynamics.

cond-mat.quant-gas↗
arXiv2011

Onset of a Pseudogap Regime in Ultracold Fermi Gases

We show, using an ab initio approach based on Quantum Monte Carlo technique, that the pseudogap regime emerges in ultracold Fermi gases close to the unitary point. We locate the onset of this regime at a value of the interaction strength corresponding to (k_{F}a)^{-1}\approx -0.05 (a - scattering length). We determine the evolution of the gap as a function of temperature and interaction strength in the Fermi gas around the unitary limit and show that our results exhibit a remarkable agreement with the recent wave-vector resolved radio frequency spectroscopy data. Our results indicate that a finite temperature structure of the Fermi gas around unitarity is more complicated and involves the presence of the phase with preformed Cooper pairs, which however do not contribute to the long range order.

cond-mat.quant-gas↗
arXiv2011

Quantum Monte Carlo study of dilute neutron matter at finite temperatures

We report results of fully non-perturbative, Path Integral Monte Carlo (PIMC) calculations for dilute neutron matter. The neutron-neutron interaction in the s channel is parameterized by the scattering length and the effective range. We calculate the energy and the chemical potential as a function of temperature at the density $\dens=0.003\fm^{-3}$. The critical temperature $\Tc$ for the superfluid-normal phase transition is estimated from the finite size scaling of the condensate fraction. At low temperatures we extract the spectral weight function $A(p,ω)$ from the imaginary time propagator using the methods of maximum entropy and singular value decomposition. We determine the quasiparticle spectrum, which can be accurately parameterized by three parameters: an effective mass $m^*$, a mean-field potential $U$, and a gap $Δ$. Large value of $Δ/\Tc$ indicates that the system is not a BCS-type superfluid at low temperatures.

nucl-th↗
arXiv2010

Generation and Dynamics of Quantized Vortices in a Unitary Fermi Superfluid

Superfluidity and superconductivity are remarkable manifestations of quantum coherence at a macroscopic scale. The dynamics of superfluids has dominated the study of these systems for decades now, but a comprehensive theoretical framework is still lacking. We introduce a local extension of the time-dependent density functional theory to describe the dynamics of fermionic superfluids. Within this approach one can correctly represent vortex quantization, generation, and dynamics, the transition from a superfluid to a normal phase and a number of other large amplitude collective modes which are beyond the scope of two-fluid hydrodynamics, Ginzburg-Landau and/or Gross-Pitaevskii approaches. We illustrate the power of this approach by studying the generation of quantized vortices, vortex rings, vortex reconnection, and transition from a superfluid to a normal state in real time for a unitary Fermi gas. We predict the emergence of a new qualitative phenomenon in superfluid dynamics of gases, the existence of stable superfluidity when the systems are stirred with velocities significantly exceeding the nominal Landau critical velocity in these systems.

cond-mat.quant-gas↗
arXiv2010

Superfluid properties of dilute neutron matter

We report results of fully non-perturbative calculations, based on Auxiliary Field Quantum Monte Carlo (AFQMC) approach, for the dilute neutron matter at the density $ρ=0.003\fm^{-3}$. Fundamental quantities which characterize the superfluid state: the single particle energy gap $Δ(T)$, and the critical temperature $\Tc$ have been determined. The large value of $Δ(0)/\Tc\approx 3.2$ indicates that the system is not a BCS-type superfluid at low temperatures.

nucl-th↗
arXiv2009

Effective Range Expansion for the Interaction Defined on the Lattice

The relation between the interaction parameters for fermions on the spatial lattice and the two-body $T$ matrix is discussed. The presented method allows determination of the interaction parameters through the relatively simple computational scheme which include the effect of finite lattice spacing. In particular the relation between the interaction parameters and the effective range expansion parameters is derived in the limit of large lattices.

nucl-th↗
arXiv2009

Nuclear tetrahedral configurations at spin zero

The possibility of the existence of stable tetrahedral deformations at spin zero is investigated using the Skyrme-HFBCS approach and the generator coordinate method (GCM). The study is limited to nuclei in which the tetrahedral mode has been predicted to be favored on the basis of non self-consistent models. Our results indicate that a clear identification of tetrahedral deformations is unlikely as they are strongly mixed with the axial octupole mode. However, the excitation energies related to the tetrahedral mode are systematically lower than those of the axial octupole mode in all the nuclei included in this study.

nucl-th↗
arXiv2008

Quantum Monte Carlo Simulations of the BCS-BEC Crossover at Finite Temperature

The Quantum Monte Carlo method for spin 1/2 fermions at finite temperature is formulated for dilute systems with an s-wave interaction. The motivation and the formalism are discussed along with descriptions of the algorithm and various numerical issues. We report on results for the energy, entropy and chemical potential as a function of temperature. We give upper bounds on the critical temperature T_c for the onset of superfluidity, obtained by studying the finite size scaling of the condensate fraction. All of these quantities were computed for couplings around the unitary regime in the range -0.5 \le (k_F a)^{-1} \le 0.2, where a is the s-wave scattering length and k_F is the Fermi momentum of a non-interacting gas at the same density. In all cases our data is consistent with normal Fermi gas behavior above a characteristic temperature T_0 > T_c, which depends on the coupling and is obtained by studying the deviation of the caloric curve from that of a free Fermi gas. For T_c < T < T_0 we find deviations from normal Fermi gas behavior that can be attributed to pairing effects. Low temperature results for the energy and the pairing gap are shown and compared with Green Function Monte Carlo results by other groups.

cond-mat.stat-mech↗
arXiv2007

Thermodynamics of a Trapped Unitary Fermi Gas

We present the first model-independent comparison of recent measurements of the entropy and of the critical temperature of a unitary Fermi gas, performed by Luo et al., with the most complete results currently available from finite temperature Monte Carlo calculations. The measurement of the critical temperature in a cold fermionic atomic cloud is consistent with a value $T_c=0.23(2)epsilon_F$ in the bulk, as predicted by the present authors in their Monte Carlo calculations.

cond-mat.stat-mech↗
arXiv2006

Spin 1/2 Fermions in the Unitary Regime: A Superfluid of a New Type

We have studied, in a fully non-perturbative calculation, a dilute system of spin 1/2 interacting fermions, characterized by an infinite scattering length at finite temperatures. Various thermodynamic properties and the condensate fraction were calculated and we have also determined the critical temperature for the superfluid-normal phase transition in this regime. The thermodynamic behavior appears as a rather surprising and unexpected melange of fermionic and bosonic features. The thermal response of a spin 1/2 fermion at the BCS-BEC crossover should be classified as that of a new type of superfluid.

cond-mat.stat-mech↗
arXiv2006

Spin 1/2 Fermions in the Unitary Regime at Finite Temperature

We have performed a fully non-perturbative calculation of the thermal properties of a system of spin 1/2 fermions in 3D in the unitary regime. We have determined the critical temperature for the superfluid-normal phase transition. The thermodynamic behavior of this system presents a number of unexpected features, and we conclude that spin 1/2 fermions in the BCS-BEC crossover should be classified as a new type of superfluid.

cond-mat.stat-mech↗
arXiv2006

Scalar Casimir effect between Dirichlet spheres or a plate and a sphere

We present a simple formalism for the evaluation of the Casimir energy for two spheres and a sphere and a plane, in case of a scalar fluctuating field, valid at any separations. We compare the exact results with various approximation schemes and establish when such schemes become useful. The formalism can be easily extended to any number of spheres and/or planes in three or arbitrary dimensions, with a variety of boundary conditions or non-overlapping potentials/non-ideal reflectors.

hep-th↗
arXiv2005

Casimir interaction between normal or superfluid grains in the Fermi sea

We report on a new force that acts on cavities (literally empty regions of space) when they are immersed in a background of non-interacting fermionic matter fields. The interaction follows from the obstructions to the (quantum mechanical) motions of the fermions caused by the presence of bubbles or other (heavy) particles in the Fermi sea, as, for example, nuclei in the neutron sea in the inner crust of a neutron star or superfluid grains in a normal Fermi liquid. The effect resembles the traditional Casimir interaction between metallic mirrors in the vacuum. However, the fluctuating electromagnetic fields are replaced by fermionic matter fields. We show that the fermionic Casimir problem for a system of spherical cavities can be solved exactly, since the calculation can be mapped onto a quantum mechanical billiard problem of a point-particle scattered off a finite number of non-overlapping spheres or disks. Finally we generalize the map method to other Casimir systems, especially to the case of a fluctuating scalar field between two spheres or a sphere and a plate under Dirichlet boundary conditions.

quant-ph↗
arXiv2005

Fermionic Casimir Effect in Case of Andreev Reflection

We describe the Fermionic Casimir effect in the case of two spherical superfluid scatterers immersed in a normal Fermi system. It is shown that due to the focusing property of Andreev reflection this new Casimir-like energy is significantly enhanced when compared to the case of normal scatterers with specular reflection.

cond-mat.supr-con↗
arXiv2003

Nuclear hydrodynamics in the inner crust of neutron stars

In the inner crust of a neutron star, due to the high density and pressure, nuclei which are still present, are immersed in a neutron superfluid. One then expects that the dynamical properties of nuclei are significantly affected. In order to estimate the magnitude of the effect associated with the presence of a superfluid medium, we formulate the hydrodynamical approach to the nuclear dynamics in the inner crust of neutron stars. We calculate the renormalized nuclear mass and the strength of the medium-induced interaction between nuclei. We argue that these effects noticeably modify the properties of the Coulomb crystal in the inner crust.

astro-ph↗
arXiv2003

In-medium ion mass renormalization and lattice vibrations in the neutron star crust

The inner crust of a neutron star consists of nuclei immersed in a superfluid neutron liquid. As these nuclei move through the fermionic medium they bring it into motion as well. As a result their mass is strongly renormalized and the spectrum of the ion lattice vibrations is significantly affected. Consequently we predict that the specific heat and the lattice thermal energy of the Coulomb crystal at these densities are noticeably modified.

astro-ph↗