Probing Holographic Superfluids with Solitons

Transcription

1 Probing Holographic Superfluids with Solitons Sean Nowling Nordita GGI Workshop on AdS4/CFT3 and the Holographic States of Matter work in collaboration with: V. Keränen, E. Keski-Vakkuri, and K.P. Yogendran

2 AdS / CM There has been much recent effort to construct strongly coupled phases of matter using the AdS/CFT correspondence superfluidity superconductivity Really want to push on these systems and say how like/unlike known systems probe calculations might be telling you about the probe, not the ground state Want an observable which is more intrinsic to the superfluid use solitons 2

3 Outline Holographic Superfluids Holographic Solitons construct dark solitons and vortices Lessons From Real World Solitons suggests how organize our exploration of holographic solutions Outlook 3

4 Holographic Superfluids Many recent attempts to use gauge/gravity duality to study 2+1 dim. condensed matter system Focus on spontaneous breaking of an Abelian symmetry AdS4 Einstein-Maxwell-Higgs gl g (R 12Λ)/2 κ2 4 F 2 q 2 /4+ DΨ 2 + m 2 Ψ 2 Hartnoll, et. al. [ and ] probe limit: κ 4 q 0 ds 2 = L 2 fdt2 z 2 (Schw-AdS b.h.) + dz2 fz 2 + dx2 z 2,f(z) =1 z 3 4

5 Begin with m 2 = 2/L 2, gauge fixed e.o.m. fr + f R zr + 2 xr + A2 t f R =0 fa t + 2 xa t R 2 A t =0 use rescaled fields R 2 Ψ z asymptotic solution: R R (1) (x)+zr (2) (x)+... A t A (0) t (x)+za (1) t (x)+... for this bulk mass we may use Neumann or Dirichlet boundary conditions: R (1) O 1, R (2) =0 A (0) t µ, A (1) t ρ tot or R (1) =0, R (2) O 2 A (0) t µ, A (1) t ρ tot 5

6 both boundary conditions display spontaneous symmetry breaking (at fixed temperature) for µ µ c charged operator has an expectation value as all charged external sources are removed can study how the order parameter behaves near μc µ O i 1 µ µc µ c can use linear response to study sound modes, probe fermions,... Can we find an observable outside linear response theory which know about the some of the short distance physics? a soliton s cores carry information about the short distance physics even at the mean field level 6

7 Holographic Dark Solitons Try to find dark soliton solutions to gravity equations fr + f R zr + 2 xr + A2 t f R =0 fa t + 2 xa t R 2 A t =0 want kink scalar field profiles external to black hole cannot be stable we will disallow any dependence on the coordinate along the soliton can add this dependence later and study the soliton s decay justified because real world kink solitons can be long lived 7

8 Aside on Numerical Methods Outside the horizon, the equations of motion are elliptic like Poisson equation 2 φ 0 = ρ Relaxation : extend to a diffusion equation with new variable t [0, ) ( t 2 )φ(t) =ρ now have a simple initial value problem: there is a flow to solution of Poisson equation the error decreases monotonically in time E(t) φ 0 φ(t) e tk2 the relaxation scheme may be implemented on a lattice topological information is contained in flow s initial conditions 8

9 Holographic Dark Solitons R A µ(1 z) <O 1 > condensate Soliton for lower dimension scalar has a greater core depletion of the charge density The fact that the depletion is not going to zero means that one cannot simply use a simple Landau-Ginzberg description 9

10 Holographic Dark Solitons R A µ(1 z) <O 2 > condensate Soliton for lower dimension scalar has a greater core depletion of the charge density The fact that the depletion is not going to zero means that one cannot simply use a simple Landau-Ginzberg description 9

11 Holographic Dark Solitons <O 1,2 > < ρ> z A R Ρ X X 1.2 A tanh(x/ξ) A + B sech 2 (x/ξ q ) Soliton for lower dimension scalar has a greater core depletion of the charge density The fact that the depletion is not going to zero means that one cannot simply use a simple Landau-Ginzberg description 9

12 Holographic Dark Solitons Ρ Ρ max <O 2 > x Ρ Ρ max Depletion Fraction <O 1 > 0.6 <O 1 > <O 2 > T T c Soliton for lower dimension scalar has a greater core depletion of the charge density The fact that the depletion is not going to zero means that one cannot simply use a simple Landau-Ginzberg description 9

13 Holographic Dark Solitons Ρ Ρ max <O 2 > x Ρ Ρ max Depletion Fraction <O 1 > 0.6 <O 1 > <O 2 > T T c Soliton for lower dimension scalar has a greater core depletion of the charge density The fact that the depletion is not going to zero means that one cannot simply use a simple Landau-Ginzberg description 9

14 Holographic Vortices Seek vortex solutions which carry nontrivial winding z 0 must include angular component of gauge field A θ, ( 0) dual to the superfluid current density j s now have 3 coupled nonlinear PDE s 0 = f 2 zr + z f z R zr + 1 ρ ρ(ρ ρ R) R( 1 f A2 t + (A θ n) 2 ρ 2 ) 0 = f 2 za t + 1 ρ ρ(ρ ρ A t ) R 2 A t 0 = z (f z A θ )+ρ ρ ( 1 ρ ρa θ ) R 2 (A θ n) 10

15 Holographic Vortices R(ρ,z) R A t (ρ,z) µ(1 z) A µ(1 z) A θ (ρ,z) <O 1 > condensate Same pattern of charge depletion fractions 11

16 Holographic Vortices R(ρ,z) R A t (ρ,z) µ(1 z) A µ(1 z) A θ (ρ,z) <O 2 > condensate Same pattern of charge depletion fractions 11

17 Holographic Vortices j s Same pattern of charge depletion fractions 11

18 Holographic Vortices Density depletion suggests that <O 1 > and that <O 2 > is BCS-like. is BEC-like Same pattern of charge depletion fractions 11

19 Holographic Vortices Density depletion suggests that <O 1 > and that <O 2 > is BCS-like. is BEC-like Same pattern of charge depletion fractions 11

20 Lessons From (non-rel.) Fermionic Superfluids (from Nishida and Son [ ]) modeled with a non-relativistic 4-Fermi interaction may study in ε expansion without keeping the dimensionful scale ( akf ) in this expansion we are tuning the scaling dimensions of fields solitons in these systems remember the microscopic structure even at the mean field level 12

21 Solitons in the Crossover Non-Relativistic BCS - BEC crossover Dark Solitons Soliton solution to gap equation display dark soliton solutions unstable -- but observable variable depletion fraction, bos. superfluid is near 100%, ferm. superfluid is much smaller has oscillations which know about kf on the BCS side Dark soliton: BEC (dot-dashed), unitarity (cont.), and BCS (dashed) (from Antezza, et. al. [ ]) 13

22 Solitons in the Crossover Non-Relativistic BCS - BEC crossover Vortices display dark soliton solutions display vortex solutions variable depletion fraction same core and tail scales in BEC, different for BCS also knows about kf on BCS side Vortex Profiles: BEC ( 1 ) and BCS ( -1 ) (from Sensarma, et. al. [cond-mat/ ]) 14

23 Solitons in the Crossover Non-Relativistic BCS - BEC crossover Vortices display dark soliton solutions display vortex solutions variable depletion fraction same core and tail scales in BEC, different for BCS also knows about kf on BCS side Vortex Profiles: BEC ( 1 ) and BCS ( -1 ) (from Sensarma, et. al. [cond-mat/ ]) 14

24 Things To Look For Suggests that we should study solitons as a function of the scaling dimension Number of independent lengthscales Friedel oscillations 15

25 Comparing Holographic Solitons Vary scaling dimension monotonic dependence in charge density depletion on Multiple lengthscales in condensate (vortex) temp. dependence ratio of lengthscales changes with Kink Dep. Frac. at T T c Friedel oscillations would give a measure of kf without using a fermion probe but... not visible (at least for T/Tc ~.5) 16

26 Comparing Holographic Solitons Vary scaling dimension monotonic dependence in charge density depletion on Multiple lengthscales in condensate (vortex) temp. dependence ratio of lengthscales changes with Friedel oscillations would give a measure of kf without using a fermion probe Ratio of core to tail length scales: <O 1 > in blue and <O 2 > in Red. but... not visible (at least for T/Tc ~.5) 16

27 Comparing Holographic Solitons Vary scaling dimension monotonic dependence in charge density depletion on Multiple lengthscales in condensate (vortex) temp. dependence ratio of lengthscales changes with Friedel oscillations would give a measure of kf without using a fermion probe Ratio of core to tail length scales: <O 1 > in blue and <O 2 > in Red. but... not visible (at least for T/Tc ~.5) 16

28 Comparing Holographic Solitons Vary scaling dimension monotonic dependence in charge density depletion on Multiple lengthscales in condensate (vortex) temp. dependence ratio of lengthscales changes with Friedel oscillations would give a measure of kf without using a fermion probe Ratio of core to tail length scales: <O 1 > in blue and <O 2 > in Red. but... not visible (at least for T/Tc ~.5) 16

29 Comparing Holographic Solitons Vary scaling dimension monotonic dependence in charge density depletion on Multiple lengthscales in condensate (vortex) temp. dependence ratio of lengthscales changes with Friedel oscillations would give a measure of kf without using a fermion probe Ratio of core to tail length scales: <O 1 > in blue and <O 2 > in Red. but... not visible (at least for T/Tc ~.5) 16

30 Comparing Holographic Solitons Vary scaling dimension monotonic dependence in charge density depletion on Multiple lengthscales in condensate (vortex) temp. dependence ratio of lengthscales changes with Friedel oscillations would give a measure of kf without using a fermion probe Ratio of core to tail length scales: <O 1 > in blue and <O 2 > in Red. but... not visible (at least for T/Tc ~.5) 16

31 Comparing Holographic Solitons Vary scaling dimension monotonic dependence in charge density depletion on Multiple lengthscales in condensate (vortex) temp. dependence ratio of lengthscales changes with Friedel oscillations would give a measure of kf without using a fermion probe Ratio of core to tail length scales: <O 1 > in blue and <O 2 > in Red. but... not visible (at least for T/Tc ~.5) 16

32 Summary and Outlook Presented dark soliton and vortex solutions in holographic superfluid the core charge density depletion is a strong function of m 2 Holographic solitons are have features which are reminiscent solitons in the BCS-BEC crossover suggests that we might vary the condensate type by changing the value of m 2 what is the analog of the crossover s unitarity regime? but no evidence of Friedel oscillations yet Follow-up: try to see any fermionic characteristics cooling to lower temperatures or may need to use external probes gravitational backreaction, transport properties, instabilities, nonrelativistic symmetries... 17