Stationary Josephson effect throughout the BCS-BEC

crossover

Pierbiagio Pieri

(work done with Andrea Spuntarelli and Giancarlo C. Strinati)

Dipartimento di Fisica, University of Camerino, Italy

Join two superconductors by a weak link (e.g. a thin normal-metal or insulating barrier). A current can flow with no potential drop across the barrier if it does not exceed a critical value .

The current is associated with a phase difference of the order parameter on the two sides of the barrier.

Josephson’s relation:

Same phenomenon occurs for two BECs separated by a potential barrier.

The stationary Josephson effect

J J

0V

| | ie ( )| | ie

cJ

sin( )cJ J

The BCS-BEC crossover

Gas of fermions interacting via an attractive potential.

• Weak attraction:Weak attraction: Cooper pairs form at low temperature according to BCS picture. Largely-overlapping pairs form and condense at the same temperature (Tc ).

• Strong attractionStrong attraction: the pair-size shrinks and pair-formation is no longer a cooperative phenomenon.

Nonoverlapping pairs (composite bosons) undergo Bose-Einstein condensation at low temperature. Pair-formation temperature and condensation critical temperature are unrelated.

BCS-BEC crossover realized experimentally with ultracold Fermi atoms by using appropriate Fano-Feshbach resonances. In this case the attractive potential is short-ranged and is parametrized completely in terms of the scattering length Dimensionless coupling parameter:

.Fa1/( )F Fk a

1/( )F Fk a-1 0 +1

BCSBCS BECBEC

In a BCS superconductor (weak attraction) the Josephson critical currentis proportional to the gap parameter:

Does this remain true through the BCS-BEC crossover?

This would imply a monotonic increase of the Josephson critical current for increasing coupling strength.

How does the Josephson’s effect changechange throughout the evolution between the two above quite different regimes?

cJ

Bogoliubov-de Gennes equations for superfluid fermions

For BCS superconductors, the microscopic treatment of the Josephson’s effect relies on solving the BdG equations with an appropriate geometry:

where

and

At T=0 the BdG equations map in the BEC limit onto the GP equation for composite bosons (Pieri & Strinati PRL 2003), thus recovering the microscopic approach to the Josephson effect for the composite bosons. The BdG equations are thus expected to provide a reliable description of the Josephson effect throughout the BCS-BEC crossover at T=0.

We assume the barrier to depend on one spatial coordinate only. Away from thebarrier in the bulk the solution for a homogeneous superfluid flowing with velocity (current ) should be recovered.

We have thus the boundary conditions:

The order parameter accumulates a phase shift across the barrier. We set:

( )2( ) | ( ) | x i xiqx x e

Geometry and boundary conditions

20

20

( ) | |

( ) | |

iqx

iqx i

x e

x e

L

0

( )x

V

x

| ( ) |x

v /q m /J nq m

( )x

x

z yJ

• Approximate and with a sequence of steps (typically 80).

• In each region the solutions of BdG eqs. are plane waves.

• Impose continuity conditions at the boundaries of each region and boundary conditions at infinity.

• Integrate over continuous energies (scattering states) + discrete sum over Andreev-Saint James bound states and enforce self-consistency on a less dense grid (typically 20).

• At convergence calculate the current from the expression:

Numerical procedure

| ( ) |x

*n n

2( ) Im v ( ) v ( )

n

Jm

r r r

( )x

Compare the numerical solution of the BdG eqs. with the solution of the GP equation for bosons of mass , scattering length , in the presence of a barrier .

2B Fa a

Check of the numerical procedure in the BEC limit

Comparison is very good!

2Bm m( ) 2 ( )BV x V x

FLk = 5.3

Comparison with delta-like barrier in BCS limit

When approaching the BCS limit with fixed barrier parameters, results for a delta-like barrier are invariably recovered: The coherence length the barrier is seen as point-like.Friedel oscillations are clearly visible in the BCS limit.

Lξ >>

0( ) ( / )F FZ Lk V E

Current vs phase relation through the crossover

At unitarity (crossover region) the Josephson current is enhanced.

Strong deviation from in the BCS limit, where a is approached.The standard Josephson’s relation is recovered in the BEC limit.

For high barriers through the whole BCS-BEC crossover.sin( )cJ J

sin( ) cos( / 2)sin( )cJ J

Critical Josephson current through the crossover

Josephson critical current controlled by Landau critical velocity (+ barrier details).

Depairing velocity:Landau criterion applied to pair-breakingexcitations.

It reduces to

in the BCS limit.

cJ

Sound velocity:Landau criterion applied to Bogoliubov-Anderson mode. Dispersion of the Bogoliubov-Anderson mode calculated from BCS-RPA.

It reduces to

in the BEC limit.

/ 2c Bq m

cJ c

0 /c Fq m k

22 2

0cq

m

when the critical velocity is determined by pair-breaking (BCS to crossover region) increases with coupling.

(where c is the Bog.-And. mode velocity) when the critical velocity is determined by excitations of sound modes decreases with coupling.

Superfluidity is most robust in the crossover region!

Preliminary experimental results

Courtesy of W. Ketterle’s group.

http://fisica.unicam.it/bcsbec

A. Spuntarelli, P.P., and G.C. Strinati, arXiv:0705.2658, to appear in PRL

Supplementary material

Evolution with the barrier height at unitarity

Gap and phase profile for different couplings

Friedel oscillations are washed out when evolving from the BCS to the BEC limit.

Suppression of the gap due to the barrier and phase difference increase monotonically from BCS to BEC limit.

cq = qcq = q

Wide barrier Intermediate barrier

Short barrier

Importance of the bound-state contribution

Current vs phase relation through the crossover

Critical Josephson current normalized to Landau critical current