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Phase diagram for a 2D spin-1/2 system with a singlet ground state Collin Broholm Johns Hopkins University & NIST

Phase diagram for a 2D spin-1/2 system with a singlet ground state Collin Broholm Johns Hopkins University & NIST. Introduction Tuning from dimer to spin order Frustrated dimers in 2D Low T spectrum Field driven critical response H-T Phase diagram MIT’s in frustrated systems

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Phase diagram for a 2D spin-1/2 system with a singlet ground state Collin Broholm Johns Hopkins University & NIST

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  1. Phase diagram for a 2D spin-1/2 system with a singlet ground stateCollin BroholmJohns Hopkins University & NIST Introduction Tuning from dimer to spin order Frustrated dimers in 2D Low T spectrum Field driven critical response H-T Phase diagram MIT’s in frustrated systems Frustrating Challenges

  2. Collaborators • PHCC • M. Bouloubasis • N. Harrison • M. Kenzelmann • D. H. Reich • M. B. Stone • H. Tao • P. Vorserwisch • I. Zaliznyak Ruthenium pyrochlores K. H. Kim N. Hur D. Adroja Q. Huang S.-W. Cheong T. G. Perring Iridium pyrochlores Satoru Nakatsuji Yo Machida Yoshiteru Maeno Toshiro Sakakibara Takashi Tayama Theory E. R. Dunkel S. Sachdev KITP Quantum Critical 2005

  3. Spin Versus Dimer Order 3D bi-partite AFM Interacting spin pairs 2×(2S+1) KITP Quantum Critical 2005

  4. Conceptual Phase Diagram for Quantum Magnets Chakravarty, Halperin, Nelson Sachdev T/J Heavy Fermion behavior Non-fermi-liquids High TC Superconductivity Quantum Critical 1/S, frustration, 1/z, H, P, x, … KITP Quantum Critical 2005

  5. hw(meV) Frustrated Magnet with spin gap KITP Quantum Critical 2005 (C4H12N2)Cu2Cl6(PHCC) Stone et al. PRB (2001)

  6. hw(meV) 1 0 l h 0 1 2D dispersion and first moment KITP Quantum Critical 2005

  7. Frustrated bonds in PHCC Green colored bonds increase ground state energy The corresponding interactions are frustrated KITP Quantum Critical 2005

  8. Bound state meets continuum KITP Quantum Critical 2005 Stone, Zalisnyak et al. (2005)

  9. T=1.77 K H=14.5 T Intensity x>300 c Field driven Quantum Criticality KITP Quantum Critical 2005

  10. Non-linear magnetization KITP Quantum Critical 2005 Stone et al to (2005)

  11. Phase diagram for frustrated 2D magnet KITP Quantum Critical 2005 Stone et al to (2005)

  12. Cross over to 2D gapless phase w/o LRO Stone et al cond-mat/0503450 KITP Quantum Critical 2005

  13. Phase diagram for frustrated 2D magnet KITP Quantum Critical 2005 Stone et al to (2005)

  14. Evidence of non-monotonic HC(T) KITP Quantum Critical 2005 Stone et al to (2005)

  15. Anomalies near QCP • Possible elements of explanation: • Cross over between 2 regimes: • 3D ordering of magnetized dimers at low T • 3D ordering of 2D bose condensate at higher T • Additional low T degrees of freedom • Nuclear spin order • Spin Peierls like lattice distortion Neel order BEC Singlet KITP Quantum Critical 2005 Stone et al to (2005)

  16. Reentrant behavior in other frustrated magnet P. Schiffer et al., PRL (1994). Y. K. Tsui et al., PRL (1999). ….and don’t forget helium! KITP Quantum Critical 2005

  17. Paramagnetic metal to singlet insulator M. Isobe and Y. Ueda JPSJ (2002) KITP Quantum Critical 2005

  18. A and B sites on vertices of corner-sharing tetrahedra A3+ site is trivalent RE with 8-fold O2- coordination B4+ site is tetravalent TM with 6-fold O2- coordination Can have both A3+ and B4+ magnetism The pyrochlore structure A2B2O7 KITP Quantum Critical 2005

  19. Bi-doping increases bandwidth causing Mott Hubbard MIT Magnet order is found only in the insulating state Electronic DOS at EF is enhanced close to the MIT 4d: Metal Insulator Transition Yoshii and Sato (1999) KITP Quantum Critical 2005

  20. Metal insulator transition in R2Ir2O7 Yanagishima and Maeno (2001) Nakatsuji et al. KITP Quantum Critical 2005

  21. Summary • PHCC: Excitation spectrum • Evidence for competing interactions • Complex cooperative singlet • Bound state decay into continuum • PHCC: phase diagram • 3D Neel phase within Bose condensed triplon phase • Cross over and/or extra degrees of freedom at QCP • Metals with frustrated magnetism • Re1-xBixRu2O7 MIT with enhanced g for x≈1 • ReIr2O7 MIT and anomalous metal for large Re KITP Quantum Critical 2005

  22. Plans for frustrated magnets • Experimental Ventures • Dynamic critical properties at H and P driven QCP • probe de-confined critical phase in D>1 • Probe wave function of impurity liberated spin-1/2 • Explore impurity induced magnetic order • Hybridization with nuclear spin and lattice at QCP • “metallic bond” singlet phases as in MgTi2O4 • Add conduction electrons • Theoretical questions • Generalized spin-Peierls effect at D>1 QCP ? • Frame work to determine from • Itinerant frustration KITP Quantum Critical 2005

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