title: Dichotomy between Attractive and Repulsive Tomonaga-Luttinger Liquids in Spin Ladders.
abstract: We present a direct NMR method to determine whether the interactions in a Tomonaga-Luttinger liquid (TLL) state of a spin-1/2 Heisenberg antiferromagnetic ladder are attractive or repulsive. For the strong-leg spin ladder compound (C_{7}H_{10}N)_{2}CuBr_{4} we find that the isothermal magnetic field dependence of the NMR relaxation rate T_{1}^{-1}(H) displays a concave curve between the two critical fields bounding the TLL regime. This is in sharp contrast to the convex curve previously reported for a strong-rung ladder, (C_{5}H_{12}N)_{2}CuBr_{4}. We show that the concavity and the convexity of T_{1}^{-1}(H), which is a fingerprint of spin fluctuations, directly reflect the attractive and repulsive fermionic interactions in the TLL, respectively. The interaction sign is alternatively determined from an indirect method combining bulk magnetization and specific heat data.
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, Boîte Postale 166, 38042, Grenoble Cedex 9, France.|Laboratory for Quantum Magnetism, Institute of Physics, Ecole Polytechnique Féderale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Jeong
authors.affiliation: Neutron Scattering and Magnetism, Laboratory for Solid State Physics, ETH, 8093 Zürich, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Schmidiger
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, Boîte Postale 166, 38042, Grenoble Cedex 9, France.
authors.forename: H
authors.initials: H
authors.lastname: Mayaffre
authors.affiliation: Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia.
authors.forename: M
authors.initials: M
authors.lastname: Klanjšek
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, Boîte Postale 166, 38042, Grenoble Cedex 9, France.
authors.forename: C
authors.initials: C
authors.lastname: Berthier
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, 31400 Toulouse, France.
authors.forename: W
authors.initials: W
authors.lastname: Knafo
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, 31400 Toulouse, France.
authors.forename: G
authors.initials: G
authors.lastname: Ballon
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, 31400 Toulouse, France.
authors.forename: B
authors.initials: B
authors.lastname: Vignolle
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, Boîte Postale 166, 38042, Grenoble Cedex 9, France.
authors.forename: S
authors.initials: S
authors.lastname: Krämer
authors.affiliation: Neutron Scattering and Magnetism, Laboratory for Solid State Physics, ETH, 8093 Zürich, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Zheludev
authors.affiliation: Laboratoire National des Champs Magnétique Intenses, LNCMI-CNRS (UPR3228), UGA, UPS, and INSA, Boîte Postale 166, 38042, Grenoble Cedex 9, France.
authors.forename: M
authors.initials: M
authors.lastname: Horvatić