1. Magnetic phases and critical points of insulators and superconductors Colloquium article: Reviews of Modern Physics, 75, 913 (2003). Reviews: http://onsager.physics.yale.edu/qafm.pdf cond-mat/0203363 Talks online: Sachdev

2. What is a quantum phase transition ? Non-analyticity in ground state properties as a function of some control parameter g T Quantum-critical Why study quantum phase transitions ? g gcgc Theory for a quantum system with strong correlations: describe phases on either side of g c by expanding in deviation from the quantum critical point. Critical point is a novel state of matter without quasiparticle excitations Critical excitations control dynamics in the wide quantum-critical region at non-zero temperatures.

3. Outline A.“Dimerized” Mott insulators with a spin gap Tuning quantum transitions by applied pressure B.Spin gap state on the square lattice Spontaneous bond order C.Tuning quantum transitions by a magnetic field 1. Mott insulators 2. Cuprate superconductors

4. (A) “Dimerized” Mott Insulators with a spin gap Tuning quantum transitions by applied pressure

5. TlCuCl 3 M. Matsumoto, B. Normand, T.M. Rice, and M. Sigrist, cond-mat/0309440.

6. TlCuCl 3 M. Matsumoto, B. Normand, T.M. Rice, and M. Sigrist, cond-mat/0309440.

7. S=1/2 spins on coupled dimers Coupled Dimer Antiferromagnet M. P. Gelfand, R. R. P. Singh, and D. A. Huse, Phys. Rev. B 40, 10801-10809 (1989). N. Katoh and M. Imada, J. Phys. Soc. Jpn. 63, 4529 (1994). J. Tworzydlo, O. Y. Osman, C. N. A. van Duin, J. Zaanen, Phys. Rev. B 59, 115 (1999). M. Matsumoto, C. Yasuda, S. Todo, and H. Takayama, Phys. Rev. B 65, 014407 (2002).

8. Weakly coupled dimers Paramagnetic ground state

9. Weakly coupled dimers Excitation: S=1 triplon (exciton, spin collective mode) Energy dispersion away from antiferromagnetic wavevector

10. Weakly coupled dimers S=1/2 spinons are confined by a linear potential into a S=1 triplon

11. TlCuCl 3 N. Cavadini, G. Heigold, W. Henggeler, A. Furrer, H.-U. Güdel, K. Krämer and H. Mutka, Phys. Rev. B 63 172414 (2001). “triplon” or spin exciton

12. Square lattice antiferromagnet Experimental realization: Ground state has long-range magnetic (Neel or spin density wave) order Excitations: 2 spin waves (magnons)

13. TlCuCl 3 J. Phys. Soc. Jpn 72, 1026 (2003)

14. 1 Neel state T=0  in  cuprates  Pressure in TlCuCl 3 Quantum paramagnet c = 0.52337(3) M. Matsumoto, C. Yasuda, S. Todo, and H. Takayama, Phys. Rev. B 65, 014407 (2002)

15. b c PHCC – a two-dimensional antiferromagnet PHCC = C 4 H 12 N 2 Cu 2 Cl 6 a c Cu Cl C N M. B. Stone, I. A. Zaliznyak, D. H. Reich, and C. Broholm, Phys. Rev. B 64, 144405 (2001).

16.   (meV) Dispersion to “chains” Not chains but planes M. B. Stone, I. A. Zaliznyak, D. H. Reich, and C. Broholm, Phys. Rev. B 64, 144405 (2001). PHCC – a two-dimensional antiferromagnet

17.   (meV) Dispersion to “chains” Not chains but planes   (meV) 0 1 0 1 h Triplon dispersion M. B. Stone, I. A. Zaliznyak, D. H. Reich, and C. Broholm, Phys. Rev. B 64, 144405 (2001). PHCC – a two-dimensional antiferromagnet

18. S. Sachdev and R.N. Bhatt, Phys. Rev. B 41, 9323 (1990). Quantitative theory of experiments and simulations: method of bond operators Operators algebra for all states on a single dimer Canonical Bose operators with a hard core constraint Spin operators on both sites can be expressed in terms of bond operators

19. Quantitative theory of experiments and simulations: method of bond operators S. Sachdev and R.N. Bhatt, Phys. Rev. B 41, 9323 (1990). A. V. Chubukov and Th. Jolicoeur, Phys. Rev. B 44, 12050 (1991).

20. Quantitative theory of experiments and simulations: method of bond operators S. Sachdev and R.N. Bhatt, Phys. Rev. B 41, 9323 (1990).

21. 3-component antiferromagnetic order parameter Field theory for quantum criticality Three triplon continuum Triplon pole Structure holds to all orders in u ~3 

22. 3-component antiferromagnetic order parameter Field theory for quantum criticality Two spin-wave continuum Structure holds to all orders in u ~3 

23. Entangled states at of order c 1/ c Triplon quasiparticle weight Z A.V. Chubukov, S. Sachdev, and J.Ye, Phys. Rev. B 49, 11919 (1994) Antiferromagnetic moment N 0 1/ c Triplon energy gap  1/ c

24. Critical coupling No quasiparticles --- dissipative critical continuum Dynamic spectrum at the critical point Field theory for quantum criticality

25. Quantum criticality described by strongly-coupled critical theory with universal dynamic response functions dependent on Triplon scattering amplitude is determined by k B T alone, and not by the value of microscopic coupling u S. Sachdev and J. Ye, Phys. Rev. Lett. 69, 2411 (1992).

26. (B) Spin gap state on the square lattice: Spontaneous bond order

27. Paramagnetic ground state of coupled ladder model

28. Can such a state with bond order be the ground state of a system with full square lattice symmetry ?

29. Need additional exchange interactions with full square lattice symmetry to move out of Neel state into paramagnet e.g. a second neighbor exchange J 2. This defines a dimensionless coupling g = J 2 / J

30. Collinear spins and compact U(1) gauge theory Key ingredient: Spin Berry Phases Write down path integral for quantum spin fluctuations

31. Collinear spins and compact U(1) gauge theory Write down path integral for quantum spin fluctuations Key ingredient: Spin Berry Phases

32. Discretize imaginary time: path integral is over fields on the sites of a cubic lattice of points a

33. Collinear spins and compact U(1) gauge theory Partition function on square lattice Modulus of weights in partition function: those of a classical ferromagnet at “temperature” g

35. Change in choice of n 0 is like a “gauge transformation” (  a is the oriented area of the spherical triangle formed by n a and the two choices for n 0 ). The area of the triangle is uncertain modulo 4  and the action is invariant under These principles strongly constrain the effective action for A a  which provides description of the large g phase

36. Simplest large g effective action for the A a  N. Read and S. Sachdev, Phys. Rev. Lett. 62, 1694 (1989). S. Sachdev and R. Jalabert, Mod. Phys. Lett. B 4, 1043 (1990). K. Park and S. Sachdev, Phys. Rev. B 65, 220405 (2002).

37. N. Read and S. Sachdev, Phys. Rev. Lett. 62, 1694 (1989).

39. For S=1/2 and large e 2, low energy height configurations are in exact one-to-one correspondence with nearest-neighbor valence bond pairings of the sites square lattice There is no roughening transition for three dimensional interfaces, which are smooth for all couplings There is a definite average height of the interface Ground state has bond order. N. Read and S. Sachdev, Phys. Rev. Lett. 62, 1694 (1989).

40. Smooth interface with average height 3/8 0 1/2 1/4 3/4 0 1/2 1/4 3/4 0 1/4 0 W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989)

41. Smooth interface with average height 5/8 1 1/2 1/4 3/4 1 1/2 1/4 3/4 1 1/4 1 W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989)

42. Smooth interface with average height 7/8 1 1/2 5/4 3/4 1 1/2 5/4 3/4 1 5/4 1 W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989)

43. Smooth interface with average height 1/8 0 1/2 1/4 -1/4 0 1/2 1/4 -1/4 0 1/4 0 W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989)

44. “Disordered-flat” interface with average height 1/2 1/2 1/4 3/4 1/2 1/4 3/4 1/4 W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989)

45. “Disordered-flat” interface with average height 3/4 1/2 3/4 1/2 3/4 W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989) 1 1 11

46. 1/4 -1/4 W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989) 0 0 00 “Disordered-flat” interface with average height 0 1/4

47. W. Zheng and S. Sachdev, Phys. Rev. B 40, 2704 (1989) 0 0 00 “Disordered-flat” interface with average height 1/4 1/4 1/2

48. Two possible bond-ordered paramagnets for S=1/2 There is a broken lattice symmetry, and the ground state is at least four-fold degenerate. N. Read and S. Sachdev, Phys. Rev. Lett. 62, 1694 (1989).

49. g 0

50. A. W. Sandvik, S. Daul, R. R. P. Singh, and D. J. Scalapino, Phys. Rev. Lett. 89, 247201 (2002) Bond order in a frustrated S=1/2 XY magnet g= First large scale numerical study of the destruction of Neel order in a S=1/2 antiferromagnet with full square lattice symmetry

51. g 0

52. g 0 S. Sachdev and R. Jalabert, Mod. Phys. Lett. B 4, 1043 (1990). K. Park and S. Sachdev, Phys. Rev. B 65, 220405 (2002).

53. g Critical theory is not expressed in terms of order parameter of either phase, but instead contains spinons interacting the a non-compact U(1) gauge force Phase diagram of S=1/2 square lattice antiferromagnet or Neel order Spontaneous bond order, confined spinons, and “triplon” excitations T. Senthil, A. Vishwanath, L. Balents, S. Sachdev and M.P.A. Fisher, submitted to Science

54. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point

55. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point

56. A. N=1, non-compact U(1), no Berry phases C. Dasgupta and B.I. Halperin, Phys. Rev. Lett. 47, 1556 (1981).

57. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point

58. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point

59. B. N=1, compact U(1), no Berry phases

60. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point

61. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point

62. C. N=1, compact U(1), Berry phases

65. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point Identical critical theories!

66. Use a sequence of simpler models which can be analyzed by duality mappings A. Non-compact QED with scalar matter B. Compact QED with scalar matter C. N=1: Compact QED with scalar matter and Berry phases D. theory E. Easy plane case for N=2 Nature of quantum critical point Identical critical theories!

67. D., compact U(1), Berry phases

68. E. Easy plane case for N=2

69. g Critical theory is not expressed in terms of order parameter of either phase, but instead contains spinons interacting the a non-compact U(1) gauge force Phase diagram of S=1/2 square lattice antiferromagnet or Neel order Spontaneous bond order, confined spinons, and “triplon” excitations T. Senthil, A. Vishwanath, L. Balents, S. Sachdev and M.P.A. Fisher, submitted to Science

70. Outline A.“Dimerized” Mott insulators with a spin gap Tuning quantum transitions by applied pressure B.Spin gap state on the square lattice Spontaneous bond order C.Tuning quantum transitions by a magnetic field 1. Mott insulators 2. Cuprate superconductors

71. (C) Tuning quantum transitions by a magnetic field 1. Mott insulators

72. 1 Neel state T=0  in  cuprates  Pressure in TlCuCl 3 Quantum paramagnet c = 0.52337(3) M. Matsumoto, C. Yasuda, S. Todo, and H. Takayama, Phys. Rev. B 65, 014407 (2002)

73. Effect of a field on paramagnet Energy of zero momentum triplon states H  0 Bose-Einstein condensation of S z =1 triplon

74. TlCuCl 3 Ch. Rüegg, N. Cavadini, A. Furrer, H.-U. Güdel, K. Krämer, H. Mutka, A. Wildes, K. Habicht, and P. Vorderwisch, Nature 423, 62 (2003).

75. TlCuCl 3 Ch. Rüegg, N. Cavadini, A. Furrer, H.-U. Güdel, K. Krämer, H. Mutka, A. Wildes, K. Habicht, and P. Vorderwisch, Nature 423, 62 (2003). “Spin wave (phonon) above critical field

76. Phase diagram in a magnetic field H 1/ Spin singlet state with a spin gap Canted magnetic order g  B H = 

77. H 1/ Spin singlet state with a spin gap Canted magnetic order 1 Tesla = 0.116 meV Related theory applies to double layer quantum Hall systems at =2 g  B H =  Phase diagram in a magnetic field

78. TlCuCl 3 M. Matsumoto, B. Normand, T.M. Rice, and M. Sigrist, cond-mat/0309440. Canted magnetic order Spin gap paramagnet

79. Phase diagram in a strong magnetic field. Magnetization = density of triplons H  Spin gap Canted magnetic order

80.  At very large H, magnetization saturates 1 Phase diagram in a strong magnetic field. Magnetization = density of triplons H Spin gap Canted magnetic order

81. M  1 1/2 Respulsive interactions between triplons can lead to magnetization plateau at any rational fraction Magnetization = density of triplons H Phase diagram in a strong magnetic field. Spin gap Canted magnetic order

82. M  1 1/2 Quantum transitions in and out of plateau are Bose-Einstein condensations of “extra/missing” triplons Magnetization = density of triplons H Partial magnetization plateau observed in SrCu 2 (BO 3 ) 2 and NH 4 CuCl 3 Phase diagram in a strong magnetic field. Spin gap Canted magnetic order

83. (C) Tuning quantum transitions by a magnetic field 2. Cuprate superconductors

84. kyky kxkx  /a 0 Insulator  ~0.12-0.14 0.055 SC 0.020 J. M. Tranquada et al., Phys. Rev. B 54, 7489 (199 6). G. Aeppli, T.E. Mason, S.M. Hayden, H.A. Mook, J. Kulda, Science 278, 1432 (1997). S. Wakimoto, G. Shirane et al., Phys. Rev. B 60, R769 (1999). Y.S. Lee, R. J. Birgeneau, M. A. Kastner et al., Phys. Rev. B 60, 3643 (1999) S. Wakimoto, R.J. Birgeneau, Y.S. Lee, and G. Shirane, Phys. Rev. B 63, 172501 (2001). (additional commensurability effects near  =0.125) T=0 phases of LSCO Interplay of SDW and SC order in the cuprates SC+SDW SDW Néel

85. kyky kxkx  /a 0 Insulator  ~0.12-0.14 0.055 SC 0.020 J. M. Tranquada et al., Phys. Rev. B 54, 7489 (199 6). G. Aeppli, T.E. Mason, S.M. Hayden, H.A. Mook, J. Kulda, Science 278, 1432 (1997). S. Wakimoto, G. Shirane et al., Phys. Rev. B 60, R769 (1999). Y.S. Lee, R. J. Birgeneau, M. A. Kastner et al., Phys. Rev. B 60, 3643 (1999) S. Wakimoto, R.J. Birgeneau, Y.S. Lee, and G. Shirane, Phys. Rev. B 63, 172501 (2001). (additional commensurability effects near  =0.125) T=0 phases of LSCO SC+SDW SDW Néel Interplay of SDW and SC order in the cuprates

86. Superconductor with T c,min =10 K kyky kxkx  /a 0  ~0.12-0.14 0.055 SC 0.020 J. M. Tranquada et al., Phys. Rev. B 54, 7489 (199 6). G. Aeppli, T.E. Mason, S.M. Hayden, H.A. Mook, J. Kulda, Science 278, 1432 (1997). S. Wakimoto, G. Shirane et al., Phys. Rev. B 60, R769 (1999). Y.S. Lee, R. J. Birgeneau, M. A. Kastner et al., Phys. Rev. B 60, 3643 (1999) S. Wakimoto, R.J. Birgeneau, Y.S. Lee, and G. Shirane, Phys. Rev. B 63, 172501 (2001). (additional commensurability effects near  =0.125) T=0 phases of LSCO SC+SDW SDW Néel Interplay of SDW and SC order in the cuprates

87. Collinear magnetic (spin density wave) order

88. Superconductor with T c,min =10 K kyky kxkx  /a 0  ~0.12-0.14 0.055 SC 0.020 T=0 phases of LSCO SC+SDW SDW Néel Use simplest assumption of a direct second-order quantum phase transition between SC and SC+SDW phases Interplay of SDW and SC order in the cuprates

89. Otherwise, new theory of coupled excitons and nodal quasiparticles L. Balents, M.P.A. Fisher, C. Nayak, Int. J. Mod. Phys. B 12, 1033 (1998). Magnetic transition in a d-wave superconductor

90. Coupling to the S=1/2 Bogoliubov quasiparticles of the d-wave superconductor Trilinear “Yukawa” coupling is prohibited unless ordering wavevector is fine-tuned. Similar terms present in action for SDW ordering in the insulator Magnetic transition in a d-wave superconductor

91. Superconductor with T c,min =10 K kyky kxkx  /a 0  ~0.12-0.14 0.055 SC 0.020 T=0 phases of LSCO SC+SDW SDW Néel H Follow intensity of elastic Bragg spots in a magnetic field Use simplest assumption of a direct second-order quantum phase transition between SC and SC+SDW phases Interplay of SDW and SC order in the cuprates Recall, in an insulator intensity would increase ~ H 2

92. Dominant effect of magnetic field: Abrikosov flux lattice

93. (extreme Type II superconductivity) Effect of magnetic field on SDW+SC to SC transition Quantum theory for dynamic and critical spin fluctuations Static Ginzburg-Landau theory for non-critical superconductivity

94. Triplon wavefunction in bare potential V 0 (x) Energy x 0 Spin gap  Vortex cores D. P. Arovas, A. J. Berlinsky, C. Kallin, and S.-C. Zhang, Phys. Rev. Lett. 79, 2871 (1997) suggested nucleation of static magnetism (with  =0) within vortex scores in a first-order transition. However, given the small size of the vortex cores, the magnetism must become dynamic as in a spin gap state. S. Sachdev, Phys. Rev. B 45, 389 (1992); N. Nagaosa and P. A. Lee, Phys. Rev. B 45, 966 (1992)

95. Energy x 0 Spin gap  Vortex cores

96. TlCuCl 3 M. Matsumoto, B. Normand, T.M. Rice, and M. Sigrist, cond-mat/0309440. Canted magnetic order Spin gap paramagnet

97. Phase diagram of SC and SDW order in a magnetic field E. Demler, S. Sachdev, and Ying Zhang, Phys. Rev. Lett. 87, 067202 (2001).

98. Phase diagram of SC and SDW order in a magnetic field

99. B. Lake, H. M. Rønnow, N. B. Christensen, G. Aeppli, K. Lefmann, D. F. McMorrow, P. Vorderwisch, P. Smeibidl, N. Mangkorntong, T. Sasagawa, M. Nohara, H. Takagi, T. E. Mason, Nature, 415, 299 (2002). See also S. Katano, M. Sato, K. Yamada, T. Suzuki, and T. Fukase, Phys. Rev. B 62, R14677 (2000).

100. Neutron scattering measurements of static spin correlations of the superconductor+spin-density-wave (SC+CM) in a magnetic field H (Tesla)

101. E. Demler, S. Sachdev, and Ying Zhang, Phys. Rev. Lett. 87, 067202 (2001). Neutron scattering observation of SDW order enhanced by superflow. Phase diagram of a superconductor in a magnetic field

102. Neutron scattering measurements of dynamic spin correlations of the superconductor (SC) in a magnetic field B. Lake, G. Aeppli, K. N. Clausen, D. F. McMorrow, K. Lefmann, N. E. Hussey, N. Mangkorntong, M. Nohara, H. Takagi, T. E. Mason, and A. Schröder, Science 291, 1759 (2001).

103. Neutron scattering measurements of dynamic spin correlations of the superconductor (SC) in a magnetic field B. Lake, G. Aeppli, K. N. Clausen, D. F. McMorrow, K. Lefmann, N. E. Hussey, N. Mangkorntong, M. Nohara, H. Takagi, T. E. Mason, and A. Schröder, Science 291, 1759 (2001).

104. Collinear magnetic (spin density wave) order

105. E. Demler, S. Sachdev, and Ying Zhang, Phys. Rev. Lett. 87, 067202 (2001). Neutron scattering observation of SDW order enhanced by superflow. Phase diagram of a superconductor in a magnetic field Prediction: SDW fluctuations enhanced by superflow and bond order pinned by vortex cores (no spins in vortices). Should be observable in STM K. Park and S. Sachdev Physical Review B 64, 184510 (2001); Y. Zhang, E. Demler and S. Sachdev, Physical Review B 66, 094501 (2002).

106. STM around vortices induced by a magnetic field in the superconducting state J. E. Hoffman, E. W. Hudson, K. M. Lang, V. Madhavan, S. H. Pan, H. Eisaki, S. Uchida, and J. C. Davis, Science 295, 466 (2002). Local density of states 1Å spatial resolution image of integrated LDOS of Bi 2 Sr 2 CaCu 2 O 8+  ( 1meV to 12 meV) at B=5 Tesla. S.H. Pan et al. Phys. Rev. Lett. 85, 1536 (2000).

107. 100Å b 7 pA 0 pA Vortex-induced LDOS of Bi 2 Sr 2 CaCu 2 O 8+  integrated from 1meV to 12meV J. Hoffman E. W. Hudson, K. M. Lang, V. Madhavan, S. H. Pan, H. Eisaki, S. Uchida, and J. C. Davis, Science 295, 466 (2002). Our interpretation: LDOS modulations are signals of bond order of period 4 revealed in vortex halo See also: S. A. Kivelson, E. Fradkin, V. Oganesyan, I. P. Bindloss, J. M. Tranquada, A. Kapitulnik, and C. Howald, cond- mat/0210683.

108. Conclusions I.Introduction to magnetic quantum criticality in coupled dimer antiferromagnet. II.Berry phases and bond order in square lattice antiferromagnets. III.Theory of quantum phase transitions provides semi- quantitative predictions for neutron scattering measurements of spin-density-wave order in superconductors; theory also proposes a connection to STM experiments. IV.Spontaneous bond order in spin gap state on the square lattice: possible connection to modulations observed in vortex halo. Conclusions I.Introduction to magnetic quantum criticality in coupled dimer antiferromagnet. II.Berry phases and bond order in square lattice antiferromagnets. III.Theory of quantum phase transitions provides semi- quantitative predictions for neutron scattering measurements of spin-density-wave order in superconductors; theory also proposes a connection to STM experiments. IV.Spontaneous bond order in spin gap state on the square lattice: possible connection to modulations observed in vortex halo.