Nov. 18, 2014 @YITP (Kyoto)

in collaboration with H. Hafermann (CEA Gif-sur-Yvette, France) A. Lichtenstein (U Hamburg, Germany)

Dual fermion approach to unconventional superconductivity and spin/charge density wave

Junya Otsuki (Tohoku U, Sendai)

仙台 (Sendai)

Outline • Extension of DMFT: dual fermion approach

• local correlation + long-range correlation • Demonstrative results for 2D Hubbard model

• AFM • Unconventional superconductivity • Phase separation • Unconventional CDW/SDW

• Further development

Strongly correlated superconductors

• Theory for Mott-Insulator, heavy fermion (formation and treatment of local moments) – Dynamical mean-field theory (DMFT)

• Theory for superconductivity – Weak-coupling expansion (RPA, FLEX) – Numerically, two-dimensional system is most challenging.

• Challenge – Unified treatment of magnetism and superconductivity – How itinerant and local natures can be dealt with at the same time

Kuramoto, Miyake 1990, Ohkawa 1992, ...

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Magnetism and Superconductivity... - appear nearby in phase diagrams - coexist

From Damascelli et al. 2003 From Shiba 2001

CePd2Si2 UGe2

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Cuprates

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From Yoshida et al. 2003

Pseudo-gap state For a review, Timusk, Statt 1999

ARPES spectra (Fermi arcs)

From Damascelli et al. 2003

Phase diagram

pseudo-gap “phase” or crossover?

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Staggered flux state / d-density wave

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c.f. Unconventional SDW/CDW discussed in URu2Si2 Ikeda, Ohashi, 1998, Varma, Zhu, 2006, Fujimoto, 2011

Numerical calculations in Hubbard model: Honerkamp et al. 2002 (fRG) Stanescu, Phillips, 2001 (Hubbard op) Macridin et al. 2004 (DCA) Lu et al, 2012 (variational cluster) Yokoyama et al. arXiv (VMC)

“hidden order” as the origin of pseudo gap

broken time-reversal symmetry Fauque et al, 2006 Shekhter et al. 2013

No evidence of the transition

Strong-coupling limit (t-J model) Kotliar, Liu, 1988 Affleck et al. 1988 Ubbens, Lee, 1992 Wen, Lee, 1995

Chakravarty et al, 2001, Nayak 2000

slave-boson MF

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Phase separation (q=0 charge instability)

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Energy argument Emery, Kivelson, 1990

Several numerical calculations support PS Kotliar et al 2002, Zitzler et al. 2002 (DMFT) Aichhorn et a. 2006 (VCA) Capone, Kotliar 2006, Macridin et al. 2006 (cluster DMFT) Werner, Millis 2007, Eckstein et al. 2007 (DMFT) Yokoyama et al. 2013 (VMC) Misawa, Imada, 2014 (mVMC)

1D t-J model Ogata et al, 1991

Random-coupling t-J model in d= ∞ JO, Vollhardt, 2013

t-J model Hubbard model

No evidence by QMC Moreo et al. 1992, Becca et al. 2000

From Misawa, Imada

controversial

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Hubbard model as a prototypical model

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to investigate Hubbard model as a prototypical model

Unresolved problem of doped Mott insulator: - peculiar spectra - d-DW / staggered flux state - Charge instability (phase separation)

to develop new theory: - Unified treatment of Mott insulator (magnetism) and superconductivity. - Extension of DMFT (approach from Mott insulator)

From Damascelli et al. 2003

motivates us ...

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Extension of DMFT: Dual fermion approach

local correlation + long-range correlation

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DMFT —Exact solution of non-trivial d=∞ limit of SCES

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Many applications... Metzner, Vollhardt 1989 Georges, Kotliar 1992 Georges et al. 1996

From Vollhardt et al. 2005

Mott insulator

Kondo lattice model

From JO et al. 2009

Heavy fermion

From JO et al. 2009

- Band-structure calculation (LDA + DMFT) - Local degrees of freedom (multi-orbital, f2 configuration, Holstein-phonon...) - Non-equilibrium

What cannot be addressed - unconventional superconductivity - quantum critical phenomena ... An extension needed

AFM Kondo insulator

T_RKKY T_K

Kondo effect vs. RKKY interaction magnetic and CDW phases

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Extension of DMFT —to incorporate non-local correlation

• Cluster extensions Maier et al. 2005 – Cellular DMFT Kotliar et al. 2001 – Dynamical cluster approximation Hettler et al. 1998 – Self-energy functional theory Potthoff 2003 finite-size effect, sign problem in QMC, ...

• Other extensions within single-site approximation

– Kusunose 2006 – Dynamical vertex approximation Toschi et al. 2007 – Dual fermion approach

Rubtsov et al. 2008, Hafermann et al. 2009 – GW + DMFT

Biermann et al. 2003, Sun, Kotliar 2004, Ayral et al. 2013 – Slezak et al. 2009 – DMFT + fRG Taranto et al. 2013

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From Potthoff 2005

From Toschi et al. 2007

combine... - local correlation by DMFT - long-range correlation (collective modes) by RPA, FLEX how to formulate? (double counting problem) dual fermion approach

spin & charge fluctuations constructed with local vertex

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Dual fermion approach I: Overview

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Rubtsov et al. 2008

Auxiliary fermion f (dual fermion) Hopping-term “decoupled”

integrate out c at each site (solve impurity problem)

Hubbard model Original lattice

Solve the dual-lattice problem

bath

c c

Approximations: 1. Retain only 2-body interactions γ 2. Perturbation expansion w.r.t γ

Dual lattice

full account of local correlations

f f

local w.r.t. c variables

f f

c c

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Dual fermion approach II: Physical viewpoint

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From Vollhardt et al. 2005

Mott insulator Heavy fermion

From JO et al. 2009

Dual-fermion lattice

for insulator

for metal (Fermi liquid)

Effective impurity problem (DMFT)

Local correlations are strictly taken into account

quasiparticles dressed by local correlations

“residual interactions”

“quasiparticle” propagator

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Dual fermion approach III: First few diagrams

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Rubtsov et al. 2008

U/t=4 (metal) U/t=8 (Mott I)

n=1, T/t=0.2

from self-consistency condition

1st-order diagram

Energy gap on the Fermi level k-dependent renormalization

2nd-order diagram

k-dependence

Higher-order diagrams give corrections to DMFT (expansion around DMFT).

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Dual fermion approach IV: Collective modes

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Hafermann et al. 2009

Ladder diagrams suppress the AF transition. (two-dimensionality is incorporated) Mermin-Wagner theorem is fulfilled.

Ladder diagram (FLEX-type diagrams)

Magnetic excitation spectrum U=4, n=1, T=0.19

Paramagnon excitations AFM fluctuations in paramagnetic state

AFM static susceptibility

DMFT: Curie-Weiss

Ladder: critical

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Dual fermion approach V: 1/d perspective

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higher order - 3-particle vertex do NOT enter in the leading correction (confirmed numerically Hafermann 2009) - Dual-fermion ladder approximation gives leading corrections to the DMFT in terms of 1/d expansion

self-consistency condition

spin & charge fluctuations s-wave pairing fluctuations (minor contribution)

our approximation

DMFT

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Demonstrative results for 2D Hubbard model

AFM Unconventional superconductivity

Charge instability (phase separation) Unconventional SDW/CDW

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Details of numerical calculations

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We investigate... - AFM - d-SC - charge instability (phase separation) - staggered flux state / d-DW

Impurity solver: Continuous-time QMC method (Rubtsov et al. 2005, Werner et al. 2006, Gull et al. 2011)

Energy cutoff

For g : N1 = 2048, …, 16384 For γ : N2, N3 = 10, …, 60

Hubbard model on a square lattice

dual-fermion self-energy

CT-HYB (Werner et al. 2006)

DMFT-like iteration

32 x 32 lattice sites

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JO, Hafermann, Lichtenstein, arXiv:1410.1246

(“quasi-2D”) Phase diagram

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U=8, doping δ=1-n Half filling, n=1

Critical regime

Mott transition in CDMFT with paramagnetic bath (e.g. Park et al. 2008)

Real AFM transition (Mermin, Wagner)

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Superconductivity I: Eigenfunctions

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Irreducible vertex for superconductivity Γpp

charge & longitudinal spin fluc.

transverse spin fluc.

Linearized BS equation

A1g

A2g [xy(x2-y2)]

B1g [x2-y2]

B2g [xy]

Eu [x]

Eu [y]

spin singlet spin triplet

even-freq.

odd-freq. even-freq.

odd-freq.

for each symmetry (spin × momentum × frequency)

(λ =1 indicates transition)

U=8t, δ=0.14, T=0.1t

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Superconductivity II: Eigenvalues

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B1g [x2-y2]

U=8t, δ=0.14

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d-SC

unstable (PS)

AFM

Phase diagram

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Phase transition? - phase separation (q=0 charge instability) - d-density wave (staggered flux state) Or just a normal metal with strong spin fluctuations.

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Phase separation (uniform charge instability)

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DMFT (U=12t) Dual Fermion (U=8t)

Mott gap

Phase separation

Chem

ical

pot

entia

l

Mott gap

two stable solutions

unstable solution

δ

µ

Mott insulator (δ= 0) + Metal (δ≠ 0)

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+: enhanced –: suppressed

Unconventional SDW/CDW

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Mean-field analysis (Ozaki 92)

+CDW +SDW – CDW

+SDW

(A)

(B)

Unconv. DW mediated by spin fluctuations

Unconventional DW

U V

Kotliar 1988 Nayak 2000

(A)

(B)

staggered flux state / d-DW

J

+SDW +CDW

+unconv.CDW +unconv.CDW +unconv.SDW

contribution to I

Effective RPA susceptibility

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Unconventional SDW/CDW

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irreducible vertex for unconventional DW

Issue: more realistic band ? (with t’)

Linearized BS equation in particle-hole channel

ordinary SDW

d-CDW

Enhanced fluctuations for d-CDW. But, d-SC is preferable than d-DW.

U=8t, δ=0.08

p-h fluctuations with q=Q (λ =1 indicates transition)

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Phase diagram

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Phase separation - consistent with DMFT, cluster DMFTs, VMC - inconsistent with QMC d-DW was not found (though fluctuations are large)

d-SC

unstable (PS)

AFM

Pure d-SC only in

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Further development

intersite interactions short-range correlations

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Further development

• DMFT: Local correlation • Dual fermion: Long-range correlation, collective modes • How to deal with short-range correlation?

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vicinity of Mott-I, frustration, spin glass, ...

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- Quantum spin glass Bray, Moore 1980, Sachdev, Ye 1993, Grempel, Rozenberg 1998, Georges et al. 2000 - 1/d fluctuations around MF Kuramoto, Fukushima 1998, JO, Kuramoto 2013 - Impurity embedded in AFM Vojta et al. 2000

For electrons systems... - Random coupling model Parcollet, Georges 1999 JO, Vollhardt 2013 - Non-random coupling model (Extended-DMFT) Smith, Si 2000, Haule et al. 2002, Sun, Kotliar 2002 GW+extendedDMFT

Coupling between spin fluctuations and local spins

DMFT for quantum spins

Dual boson approach

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U U U U

U U

U U U

U

Short-range correlations: Feedback from spin fluctuations to the effective impurity

Intersite interactions beyond MF: Perturbation theory w.r.t. auxiliary field (dual boson) Treating χq0 and Jq on equal footing

Rubtsov et al. 2012

dual-boson self energy dual-boson propagator

CT-QMC for bosonic bath Werner, Millis 2007, 2010 JO, 2013

Summary

• Extension of DMFT – Local correlation by DMFT – Long-range correlation by FLEX-type diagrams in dual fermion

• Demonstrative results – AFM: paramagnon excitations – d-SC at 0.15 ≲ δ ≲ 0.18 – Phase separation at δ ≲ 0.15 – Large fluctuations for d-DW, but no transition

• Issues – Reasonable approximation to the vertices (low-energy behavior of gamma)

Microscopic derivations of Fermi liquid

• Future investigations – Short-range correlation by spin-boson coupling in dual boson – Heavy-fermion superconductivity: application to the Kondo lattice

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JO, Hafermann, Lichtenstein, arXiv:1410.1246