arXiv subjects

N. Fukushima

Publications and source records attributed to N. Fukushima.

6 recordsLinked to original sources

arXiv2006

Superfluid density and condensate fraction in the BCS-BEC crossover regime at finite temperatures

The superfluid density is a fundamental quantity describing the response to a rotation as well as in two-fluid collisional hydrodynamics. We present extensive calculations of the superfluid density ρ_s in the BCS-BEC crossover regime of a uniform superfluid Fermi gas at finite temperatures. We include strong-coupling or fluctuation effects on these quantities within a Gaussian approximation. We also incorporate the same fluctuation effects into the BCS single-particle excitations described by the superfluid order parameter Δand Fermi chemical potential μ, using the Nozières and Schmitt-Rink (NSR) approximation. This treatment is shown to be necessary for consistent treatment of ρ_s over the entire BCS-BEC crossover. We also calculate the condensate fraction N_c as a function of the temperature, a quantity which is quite different from the superfluid density ρ_s. We show that the mean-field expression for the condensate fraction N_c is a good approximation even in the strong-coupling BEC regime. Our numerical results show how ρ_s and N_c depend on temperature, from the weak-coupling BCS region to the BEC region of tightly-bound Cooper pair molecules. In a companion paper by the authors (cond-mat/0609187), we derive an equivalent expression for ρ_s from the thermodynamic potential, which exhibits the role of the pairing fluctuations in a more explicit manner.

cond-mat.mtrl-sci↗

arXiv2006

Pairing fluctuations and the superfluid density through the BCS-BEC crossover

We derive an expression for the superfluid density of a uniform two-component Fermi gas through the BCS-BEC crossover in terms of the thermodynamic potential in the presence of an imposed superfluid flow. Treating the pairing fluctuations in a Gaussian approximation following the approach of Nozières and Schmitt-Rink, we use this definition of $ρ_s$ to obtain an explicit result which is valid at finite temperatures and over the full BCS-BEC crossover. It is crucial that the BCS gap $Δ$, the chemical potential $μ$, and $ρ_s$ all include the effect of fluctuations at the same level in a self-consistent manner. We show that the normal fluid density $ρ_n \equiv n - ρ_s$ naturally separates into a sum of contributions from Fermi BCS quasiparticles ($ρ^F_{n}$) and Bose collective modes ($ρ^B_{n}$). The expression for $ρ^F_{n}$ is just Landau's formula for a BCS Fermi superfluid but now calculated over the BCS-BEC crossover. The expression for the Bose contribution $ρ^B_{n}$ is more complicated and only reduces to Landau's formula for a Bose superfluid in the extreme BEC limit, where all the fermions have formed stable Bose pairs and the Bogoliubov excitations of the associated molecular Bose condensate are undamped. In a companion paper, we present numerical calculations of $ρ_s$ using an expression equivalent to the one derived in this paper, over the BCS-BEC crossover, including unitarity, and at finite temperatures.

cond-mat.other↗

arXiv2005

On the evaluation of matrix elements in partially projected wave functions

We generalize the Gutzwiller approximation scheme to the calculation of nontrivial matrix elements between the ground state and excited states. In our scheme, the normalization of the Gutzwiller wave function relative to a partially projected wave function with a single non projected site (the reservoir site) plays a key role. For the Gutzwiller projected Fermi sea, we evaluate the relative normalization both analytically and by variational Monte-Carlo (VMC). We also report VMC results for projected superconducting states that show novel oscillations in the hole density near the reservoir site.

cond-mat.str-el↗

arXiv2004

Exchange Interactions and High-Energy Spin States in Mn_12-acetate

We perform inelastic neutron scattering measurements on the molecular nanomagnet Mn_12-acetate to measure the excitation spectrum up to 45meV (500K). We isolate magnetic excitations in two groups at 5-6.5meV (60-75K) and 8-10.5meV (95-120K), with higher levels appearing only at 27meV (310K) and 31meV (360K). From a detailed characterization of the transition peaks we show that all of the low-energy modes appear to be separate S = 9 excitations above the S = 10 ground state, with the peak at 27meV (310K) corresponding to the first S = 11 excitation. We consider a general model for the four exchange interaction parameters of the molecule. The static susceptibility is computed by high-temperature series expansion and the energy spectrum, matrix elements and ground-state spin configuration by exact diagonalization. The theoretical results are matched with experimental observation by inclusion of cluster anisotropy parameters, revealing strong constraints on possible parameter sets. We conclude that only a model with dominant exchange couplings J_1 ~ J_2 ~ 5.5meV (65K) and small couplings J_3 ~ J_4 ~ 0.6meV (7K) is consistent with the experimental data.

cond-mat.str-el↗

arXiv2004

Exchange constants and spin dynamics in Mn12-acetate

We have obtained new inelastic neutron scattering (INS) data for the molecular magnet Mn12-acetate which exhibit at least six magnetic peaks in the energy range 5-35 meV. These are compared with a microscopic Heisenberg model for the 12 quantum spins localised on the Mn ions, coupled by four inequivalent magnetic exchange constants. A fit to the magnetic susceptibility under the constraint that the spin of the ground state be S=10 yields two dominant exchange constants of very similar value, J1 ~ J2 ~ 65 K (~ 5.5 meV), and two smaller exchange constants J3 and J4. We compute the low-lying excitations by exact numerical diagonalisation and demonstrate that the parameters determined from the ground state and susceptibility fit provide qualitative agreement with the excitations observed by INS.

cond-mat.str-el↗

arXiv1999

Spontaneous polarization of the Kondo problem associated with the higher-spin six-vertex model

We study the multi-channel Kondo model associated with an integrable higher-spin analogue of the anti-ferroelectric six-vertex model, which is constructed by inserting spin 1/2 to spin 1 lines: $... C^3 \otimes C^3 \otimes C^2 \otimes C^3 \otimes C^3 ... $. We formulate the problem in terms of representation theory of quantum affine algebra $U_q(hat{sl}_2)$. We derive an exact formula for the spontaneous staggered polarization for our model, which corresponds to Baxter`s formula for the six-vertex model.

solv-int↗