ORBITAL SELECTIVITY AND HUNDS Laura Fanfarillo PHYSICS IN - - PowerPoint PPT Presentation
ORBITAL SELECTIVITY AND HUNDS Laura Fanfarillo PHYSICS IN IRON-BASED SC FROM FERMI LIQUID TO NON-FERMI LIQUID High Temperature Bad Metal Strong Fermi Liquid Correlation Low Temperature Tuning parameter Unconventional SC emerges at
Laura Fanfarillo
FROM FERMI LIQUID TO NON-FERMI LIQUID
Low Temperature High Temperature
Unconventional SC emerges at low temperature from a state that is far from an ideal metal
Tuning parameter
FROM FERMI LIQUID TO NON-FERMI LIQUID : CUPRATES Unconventional SC emerges at low temperature from a state that is far from an ideal metal Physics of a doped Mott Insulator
A3C60 Ruthenates, Iridates … … and Iron based SC
Itinerant Electrons Picture Fermi-Surface Instabilities (Nesting) Localized Electrons Picture Magnetic SuperExchange
Hund’s Physics
Orbital Selective Mott Physics
Small Crystal Field Splitting + Hund’s coupling
DeMedici et al PRL 107 2011, DeMedici et al, PRL 112 2014, Fanfarillo et al PRB 92 2015 …
Quasiparticle Spectral Weight Suppressed Z~1/m* increasing of correlation Charge Fluctuations Suppressed: localization of the electrons Spin Fluctuations Enhanced atoms are locally spin polarized Despite the conduction band is half-filled the system is insulating because of the strong Coulomb repulsion
U >> t Z=1 FL - Metal Z=0 Correlated electrons - Insulator
High Spin Phase Correlated bad metal close to the Mott insulator Far from half-filling (n ≠1) :
Pair hopp pping ng Hund’s co coupli pling ng tb tb (hoppi
ng term) m) In Intra ra-or
bital al repul ulsio sion In Inter er-or
tal repul ulsio sion Interactions are local and satisfy rotational invariance: 𝑉′ = 𝑉 − 2𝐾𝐼 𝑉 and 𝐾𝐼 are free parameters
Quasiparticle Spectral Weight 𝑎(𝑉, 𝐾𝐼)
2(4) el/3orb Hund induces correlated metal state
𝑒𝑎 𝑒𝐾𝐼
Fanfarillo & Bascones, PRB 92(2015)
Quasiparticle Spectral Weight 𝑎(𝑉, 𝐾𝐼) Strong doping dependence: 2(4) el/3orb Hund induces correlated metal state
DeMedici et al PRL 107 (2011)
6 el in 5 orb U = W Hund’metal linked to the half-filled n=5 Mott insulator doping asymmetry around n=6 Hund’s coupling induced high spin configuration
Fanfarillo & Bascones, PRB 92(2015)
6 el in 5 orb U = W Hund’metal linked to the half-filled n=5 Mott insulator doping asymmetry around n=6 Hund’s coupling induced high spin configuration
Fanfarillo & Bascones, PRB 92(2015)
Quasiparticle weight and charge fluctuations: Correlation vs Localization
Suppression of coherence due to suppression of hopping processes which involve intraorbital double occupancy Enhancement of charge fluctuations due to hopping processes which involve parallel spins to an empty orbital
𝐹𝑗𝑜𝑢𝑠𝑏↑↓ = 𝑉 + 𝑜 − 1 𝐾𝐼 𝐹↑↑ = 𝑉 − 3𝐾𝐼
Fanfarillo & Bascones, PRB 92 (2015)
As the double occupancies are suppressed:
In the polarized state the effective interorbital interaction between the electrons decreases. It vanishes at 𝐾𝐼 = 𝑉/3.
Fanfarillo & Bascones, PRB 92(2015)
EFFECTIVE MASS
Each orbital behaves as a doped Mott insulator
De Medici et al PRL 112 (2014)
KFe2As2
5.5el-5orb
BaFe2As2
6el-5orb
proportional to the orbital filling
Consequences: local spin polarization and orbital decoupling
IBS collection of five decoupled single-band doped Mott insulator Correlations increase reducing the number of electrons in d-bands: KFe2As2 is much more correlated than BaFe2As2
ORBITAL SELECTIVITY AND HUND’S PHYSICS IN THE PHASE DIAGRAM OF IBS
Project interacting multiorbital Hamilonian into low-energy model for IBS Orbital selective character of spin fluctuations
Fanfarillo et al. PRB 91 (2015), Christenses et al. PRB 93 (2016) Fanfarillo et al arXiv 1605.02482 ...
Try to figure out if local correlations can explain the phase diagram of IBS Orbital selective SC …
DeMedici et al arXiv 1609.01303 Fanfarillo et al arXiv 1609.06672 ...
Structural transition takes place before/simultaneously to the magnetic one: Several experimental probes revealed x,y anisotropy above the magnetic transition not only in the lattice parameter but also in the electronic properties: NEMATIC PHASE
Resistivity anisotropy measurements
J-H Chu at al. Science 329 (2010)
Structural transition takes place before/simultaneously to the magnetic one: Several experimental probes revealed x,y anisotropy above the magnetic transition not only in the lattice parameter but also in the electronic properties: NEMATIC PHASE
Possible origin of “nematic phase”:
Classical “chicken and egg problem” All three types of order (structural, orbital and spin-driven nematic) are very entangled no matter which drives the nematic instability.
What drives nematic order in iron-based superconductors?
R.M. Fernandes et al. NATURE PHYSICS | VOL 10 | FEBRUARY 2014
Enigmatic nematic
Anisotropy from the lattice parameters (odd!) Anisotropy from the orbital filling Anisotropy from spin fluctuations along x,y
Sun et al Nat. Commun. 7 (2016)
Sizeble SDW fluctuations but NO magnetic long range ordered phase
Is the charge degree of freedom the driver? Can local correlations induce a nematic phase transition?
From ARPES, Quantum oscillations, X ray FeSe ~ 𝑉 = 3.5 eV and 𝐾𝐼/𝑉 = 0.20 Compute the Response of the system to orbital perturbations modulated in k-space: Orbital Nematic Parameter: Linear response:
Lift the degeneracy of the nn hopping Onsite ferro-orbital d-wave bond order 3 Orbital Orders considered in literature: Lift the degeneracy of the second neighbor hopping Sign-change orbital order
Chubukov et al. arxiv 1602.05503
No divergence = no phase transition Interactions strongly suppress OFO order: Suppression in correspondence of the entrance in the Hund Metal region. SCO order independent by U
𝐾𝐼/𝑉 = 0.20
𝐾𝐼/𝑉 = 0.20
Sign-changing orbital order small occupation imbalance between zx and yz orbitals not suppressed by interactions!
Local correlations cannot drive alone nematic transition Correlations constrain possible orbital orders
Onsite ferro-orbital ordering strongly suppressed by interactions Sign-changing orbital order = small occupation imbalance between zx and yz orbitals = not suppressed by Hund’s coupling.
Fanfarillo et al. arxiv 1609.06672
From RG Analysis: Nematicity in the Pomeranchuk d-wave assisted by spin fluctuations
Chubukov et al. arxiv 1602.05503
New route to nematicity: anisotropy in the orbital effective mass
Anisotropy in the orbital mass is induced by the orbital order perturbation. Enhanced response at the entrance of the Hund Metal. New route to nematicity: anisotropy in the orbital effective mass
In the NEMATIC state finite splitting appears between zx and yz bands at the symmetry points. In the PARAMAGNETIC state zx and yz are degenerate = NO splitting at the symmetry points
Fanfarillo et al. arxiv 1605.02482
In the NEMATIC state finite splitting appears between zx and yz bands at the symmetry points.
Given an orbital perturbation the naive splitting expected at the and M point are: Interactions renormalize the band structure (via Z xz/yz anisotropy) and can modify the bare splitting
In the PARAMAGNETIC state zx and yz are degenerate = NO splitting at the symmetry points
Local Correlations modify the orbital splitting:
Induce k-dependence, drive sign change …
Fanfarillo et al. arxiv 1609.06672
Hund’s coupling induces anisotropy in the correlation strength of zx and yz orbitals Hund’s physics modifies the magnitude of these splittings, their relative value and even their sign. From ARPES: Hole/electron sign change orbital polarization observed in FeSe interpreted as a self- energy effect of a low energy orbital selective model
Fanfarillo et al. arxiv 1605.02482
Only orbital orders that do NOT create large occupation unbalance survive to the correlations Hund’s induce anisotropy in the effective masses of zx and yz orbitals.
This anisotropy affects the renormalization of the band structure, leading to distinctive signatures in different experimental probes including ARPES.
Important insights for low-energy modeling of IBS
Fanfarillo et al. arxiv 1609.06672
M.Capone ICMM-MADRID
B.Valenzuela Sapienza - ROME
Paris-Sud Orsay
Parent Compound: SDW bad metal Multiband SC System: 3d Iron orbitals
Q - SDW vector
Nodless gap: s symmetry
Contrasting evidences for correlation strength
ARPES, Q. Osc. with respect DFT
Itinerant electron vs Localized electrons picture
weak strong
→ Onsite perturbation Onsite splitting 𝜀𝜗 = 𝜗𝑦𝑨 − 𝜗𝑧𝑨
→ Susceptibility
shrinking of the Fermi Surfaces
Fanfarillo et al PRB 91 2015, Fanfarillo et al. arXiv: 1605.02428
Project interacting multiorbital Hamilonian into low-energy model for IBS