Bruce Normand, Dr. 

e-mail:               normand@ubaclu.unibas.ch

Research Projects 

Heisenberg Antiferromagnets

 D. Loss, B. Normand, J. Kyriakidis

DESCRIPTION:
Field-theoretic investigations of the low-dimensional antiferromagnetic
Heisenberg model in the low-energy limit in terms of the non-linear sigma
model which explicitly includes the effect of magnetic fields
and the conservation of e.g. the total spin.
We investigate applications to spin ladder materials and
high temperature superconductivity materials.
Study of mesoscopic fluctuations of the spin stiffness and the transverse
magnetic susceptibility in antiferromagnets. 

Dynamical Properties of an Antiferromagnet near the 
Quantum Critical Point: Application to LaCuO2.5

B. Normand and T. M. Rice

DESCRIPTION:
For a system of two-chain spin ladders, the ground state for weak
interladder coupling is the spin-liquid state of the isolated ladder, but
is an ordered antiferromagnet (AF) for sufficiently large interactions. We
generalize the bond-operator mean-field theory to describe both regimes, and
to focus on the transition between them. In the AF phase near the quantum
critical point (QCP) we find both spin waves and a low-lying but massive
amplitude mode which is absent in a conventional AF. The static
susceptibility has the form $\chi(T) = \chi_0 + a T^2$, with $\chi_0$ small
for a system near criticality. We consider the dynamical properties to
examine novel features due to the presence of the amplitude mode, and compute
the dynamic structure factor. 

Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System

B. Normand, K. Penc, M. Albrecht and F. Mila

DESCRIPTION:
We present a theoretical study of the magnetic phase diagram of the
frustrated coupled ladder structure realized recently in several materials.
This system displays a nondegenerate spin-gap state in the dimer limit and
an infinitely degenerate spin-gap state in the regime of weakly-coupled
zig-zag chains. Between these we demonstrate the existence of gapless,
magnetically ordered regions whose order is antiferromagnetic close to the
honeycomb lattice limit, and incommensurate along the chains when all three
magnetic interactions compete. We discuss the properties and location in the
phase diagram of several cuprate and particularly vanadate compounds
displaying the given conformation but differing interaction parameters.

List of Publications: 

  • Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.

    • B. Normand, K. Penc, M. Albrecht, F. Mila.
    Phys. Rev. B 56, R5736 (1997).
     
  • Dynamical Properties of an Antiferromagnet near the Quantum Critical Point: Application to LaCuO2.5.

    • B. Normand, T.M. Rice.
    Phys. Rev. B 56, 8760 (1997).
     
  • Magnetic Properties of the Ladder Compound LaCuO2.5.

    • B. Normand, T.M. Rice.
    Physica C 282-287, 1113 (1997).
     
  • Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.

    • B. Normand, K. Penc, M. Albrecht, F. Mila.
    Physica C 282-287, 1117 (1997).

    Electronic and Magnetic Structure of LaCuO2.5.
    B. Normand, T.M. Rice. Z. Phys. B 103, 181 (1997).
     

  • LaCuO2.5: Dynamical Properties of an Antiferromagnet near the Quantum Critical Point.

    • B. Normand, T.M. Rice.
    Physica B, to appear.
     
  • Magnetic Properties of the Coupled Ladder System MgV2O5.

    • P. Millet, C. Satto, J. Bonvoisin, B. Normand, K. Penc, M. Albrecht, F. Mila.
    Phys. Rev. B, to appear.

    Talks, Seminars, Posters 

  • Spin Ladders: Novel Phenomena in Low-Dimensional Quantum Magnets.

    • Seminar, Uni. Augsburg, Germany, Jan 7, 1997.
     
  • LaCuO2.5: an Antiferromagnet near the Quantum Critical Point.

    • Seminar, MPI Garching, Germany, Jan 9, 1997.
     
  • LaCuO2.5: Dynamical Properties of an Antiferromagnet near the Quantum Critical Point.

    • Poster, NATO ASI, Geilo, Norway, Apr 2-12, 1997.
     
  • Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.

    • Poster, NATO ASI, Geilo, Norway, Apr 2-12, 1997.
     
  • Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.

    • Seminar, Uni. K\"oln, Germany, Apr 21, 1997.
     
  • Spin Ladders: Novel Phenomena in Low-Dimensional Quantum Magnets.

    • Seminar, PSI Villigen, Switzerland, Apr 30, 1997.
     
  • Spin Ladders: Novel Phenomena in Low-Dimensional Quantum Magnets.

    • Seminar, HKUST, Hong Kong, May 7, 1997.
     
  • LaCuO2.5: an Antiferromagnet near the Quantum Critical Point.

    • Seminar, Uni. Tokyo, Japan, May 12, 1997.
     
  • Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.

    • Seminar, ISSP Tokyo, Japan, May 13, 1997.
     
  • Spin Ladders: Novel Phenomena in Low-Dimensional Quantum

    • Magnets LaCuO2.5: an Antiferromagnet near the Quantum Critical Point.
    Seminars, APCPT Seoul, S. Korea, May 16, 1997.
     
  • Spin Ladders: Novel Phenomena in Low-Dimensional Quantum

    • Magnets LaCuO2.5: an Antiferromagnet near the Quantum Critical Point
    Vanadates - Spin Plaquettes, Networks, Ladders and Chains
    Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.
    Seminars, POSTECH, Pohang, S. Korea, May 19-23, 1997.
     
  • LaCuO2.5: an Antiferromagnet near the Quantum Critical Point.

    • Seminar, Uni. Tohoku, Japan, May 27, 1997.
     
  • Spin Ladders: Novel Phenomena in Low-Dimensional Quantum Magnets.

    • Seminar, Uni. Kyoto, Japan, May 29, 1997.
     
  • Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.

    • Seminar, Uni. Osaka, Japan, May 30, 1997.
     
  • LaCuO2.5: Dynamical Properties of an Antiferromagnet near the Quantum Critical Point.

    • Poster, CNRS GDR, Aussois, France, June 2-4, 1997.
     
  • Phase Diagram of the S = 1/2 Frustrated Coupled Ladder System.

    • Poster, CNRS GDR, Aussois, France, June 2-4, 1997.
     
  • LaCuO2.5: an Antiferromagnet near the Quantum Critical Point.

    • Seminar, MPIPKS Dresden, Germany, July 2, 1997.
     
  • Phase Diagram of the S=1/2 Frustrated Coupled Ladder System.

    • Seminar, UC Santa Barbara, USA, Aug 5, 1997.
     
  • Phase Diagram of the S=1/2 Frustrated Coupled Ladder System.

    • Seminar, Uni. Yale, USA, Aug 13, 1997.
     
  • LaCuO2.5: Dynamical Properties of an Antiferromagnet near the Quantum Critical Point.

    • Seminar, NEC Princeton, USA, Aug 14, 1997.
     
  • Phase Diagram of the S=1/2 Frustrated Coupled Ladder System.

    • Seminar, Bell Labs, NJ, USA, Aug 15, 1997.
     
  • LaCuO2.5: Dynamical Properties of an Antiferromagnet near the Quantum Critical Point.

    • Poster, ICNS, Toronto, Canada, Aug 17-21, 1997.
     
  • LaCuO2.5: Dynamical Properties of an Antiferromagnet near the Quantum Critical Point.

    • Seminar, Stanford Uni, USA, Aug 26, 1997.
     
  • Phase Diagram of the S=1/2 Frustrated Coupled Ladder System.

    • Seminar, UBC Vancouver, Canada, Sept 10, 1997.
     
  • LaCuO2.5:Dynamical Properties of an Antiferromagnet near the Quantum Critical Point.

    • Poster, SWSNM, Les Diablerets, Switzerland, Sept 29 - Oct 1, 1997.
     
  • Phase Diagram of the S=1/2 Frustrated Coupled Ladder System.

    • Poster, SWSNM, Les Diablerets, Switzerland, Sept 29 - Oct 1, 1997.
     
  • Linear Magnetization Characteristics of Low-Dimensional Spin Systems.

    • Contributed talk C30.2 at the 1998 March Meeting, Los Angeles, California, 16-20 March 1998.