SNS pictureSNS picture

““Recent Developments in Recent Developments in the Study of Strongly the Study of Strongly Correlated Electronic Correlated Electronic

Systems”Systems”E. Dagotto, UT-ORNLE. Dagotto, UT-ORNL

SNS 2006-8, HFIRSNS 2006-8, HFIR

Nanocenter (CNMS)Nanocenter (CNMS)

March 2005

Joint InstitutesJoint Institutes(JIAM,JINS)(JIAM,JINS)

OrganizationOrganization

• Strong correlation in nanoscopic systems: transport in molecules and quantum dots.

• Manganites: recent simulations of realistic models. Cuprates: recent simulations of phenomenological models.

UT Strong InteractionStrong Interaction

UT Bulk (Mn,Cu oxides)Bulk (Mn,Cu oxides)

UT Nano systemsNano systems

A. FeiguinA. Feiguin K. Al-HassaniehK. Al-Hassanieh G. MartinsG. Martins

C. SenC. Sen G. AlvarezG. Alvarez

+ A. Moreo, M. Mayr, and S. Yunoki+ A. Moreo, M. Mayr, and S. Yunoki

(1) (1) Molecular conductorsMolecular conductorsand quantum dotsand quantum dots

Key role of phonons in molecular conductorsKey role of phonons in molecular conductors

Kondo effect observed in molecules with net spin via Kondo effect observed in molecules with net spin via enhancement in conductance (Coulombic effects cannot enhancement in conductance (Coulombic effects cannot

be neglected in small molecules)be neglected in small molecules)

DMRG/Lanczos for Transport DMRG/Lanczos for Transport in SCES systems in SCES systems (Al-Hassanieh et al., PRB B73, (Al-Hassanieh et al., PRB B73,

195304 (2006); Schneider and Schmitteckert, preprint.)195304 (2006); Schneider and Schmitteckert, preprint.)Non interactingNon interacting

One DotOne Dot

Current vs. timeCurrent vs. time

New results: quantum interferencesNew results: quantum interferences

Anomalous cancellation of conductance in arrays of quantumAnomalous cancellation of conductance in arrays of quantumdots, Center of Mass phonons in molecules, and in T-geometries.dots, Center of Mass phonons in molecules, and in T-geometries.In progress: finite bias -> currents in Mott insulators, etc.In progress: finite bias -> currents in Mott insulators, etc.

G. Martins et al., PRL 05G. Martins et al., PRL 05K. Al-Hassanieh et al., PRL 05K. Al-Hassanieh et al., PRL 05G. Martins et al., PRL 06G. Martins et al., PRL 06

+A. Feiguin, C. Busser, et al. +A. Feiguin, C. Busser, et al.

(2,3) Complex materials, phase (2,3) Complex materials, phase competitioncompetition

Mitchell, Raveau et al.

Manganites

Cava et al.ORNL group

Cobaltites

+ vast evidence of+ vast evidence ofinhomogeneous inhomogeneous states in manganitesstates in manganitesand cupratesand cuprates(Cheong, De Teresa, (Cheong, De Teresa, Fernandez-Baca, Dusenhofer, Fernandez-Baca, Dusenhofer, Hennion, Fischer,, Zheng, Hennion, Fischer,, Zheng, Khomskii, Argyriou, Khomskii, Argyriou, Perring, Radaelli, …)Perring, Radaelli, …)

Intrinsic softness!!Intrinsic softness!!E.D., Science 2005E.D., Science 2005

Colossal Magneto Resistance Colossal Magneto Resistance (CMR)(CMR)

Resistivity of Mn-oxideschanges by 10 orders ofmagnitude at low T (1994)

Big Dreams: Potential applications inread sensors. GMR already in computers.Critical temperatures must be raised.

(1) Phase competition FM Metal vs. CO/AF Insulator is the basis of the phenomenon.(2) Simulations of simple models (resistor networks) show CMR.(3) Both strong Hund coupling and JT phonons needed.

Very recent effort at UT/ORNL aims to observe CMR in doubleexchange models of Mn oxides. It exploits (i) ORNL computers facilities with thousands of processors for parallel computing, (ii) new algorithms TPEM.

Special Case: CMRSpecial Case: CMR

T*

SG

T

W

FMCO

mixed

S. Yunoki et al., PRL 98, A. Moreo et al. Science 99; S. Yunoki et al., PRL 98, A. Moreo et al. Science 99; E.D. et al., Phys.Rep. 01E.D. et al., Phys.Rep. 01

Final attack: realistic models, one- and two-orbital DE with phonons (C. Sen, G. Alvarez, et al., PRB, to appear)

Large peakLarge peakeven in cleaneven in cleanlimit at low nlimit at low n

Large MRLarge MR

Importance of Coulomb,Importance of Coulomb,phonons, and disorder!phonons, and disorder! TwoTwo

orbitalsorbitals

Recent results, involving two phases Recent results, involving two phases with different spin/charge orderwith different spin/charge order..

One-orbital model,One-orbital model,realistic density, FM metalrealistic density, FM metalvs. CO/AF competition.vs. CO/AF competition.

Origin of glassy state? Patches of SC above TcOrigin of glassy state? Patches of SC above Tc? ?

(Alvarez et al., PRB 71, 014514 (2005); M. Mayr et al., PRB 06).(Alvarez et al., PRB 71, 014514 (2005); M. Mayr et al., PRB 06).

Coulombiccenters, as inSr++. Each provides 1h.

T*

AF

eeii

SC

Giant responses?Giant responses?Giant proximity effectGiant proximity effect

Strain?Strain?

ConclusionsConclusions

• Strong interactions are important in transport Strong interactions are important in transport in small molecules and dots. Bulk in small molecules and dots. Bulk Nano? Nano?

• Interesting physics arises from a combination Interesting physics arises from a combination of Coulombic, phononic, and disordering of Coulombic, phononic, and disordering effects. We need a ``Theory of Everything’’.effects. We need a ``Theory of Everything’’.

• Electronic softness, self-organization, several d. Electronic softness, self-organization, several d. of f. simultaneously active: Cause of CMR? of f. simultaneously active: Cause of CMR? Yes. Cause of high-Tc? Yes. Cause of high-Tc?