Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U2: Quantum Magnets in High Magnetic Fields |
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Sponsoring Units: DCMP Chair: Igor Zaliznyak, Brookhaven National Laboratory Room: Baltimore Convention Center Ballroom III |
Thursday, March 16, 2006 8:00AM - 8:36AM |
U2.00001: Spinons, Solitons, and Breathers in Quasi-One-Dimensional Magnets Invited Speaker: By scattering neutrons from coordination polymer magnets, we contrast the effects of a uniform and a staggered magnetic field on the quantum critical state of a spin-1/2 chain. In a partially magnetized state of copper pyrazine dinitrate (CuPzN) we find bounded spectral continua indicating that neutrons scatter from spin-1/2 quasi-particle pairs [1]. The complex boundaries including an incommensurate soft spot result from a field induced shift in the Fermi points for these quasi-particles. The measurements indicate that the magnetized state of CuPzN remains quantum critical. Copper benzoate [2] and CuCl$_{2}^{.}$2(dimethylsulfoxide) (CDC) [3] differ from CuPzN in that there are two spins per unit cell along the spin chain. Rather than continuous spectra, we find resolution limited gapped excitations when these materials are subject to high fields. So with two spins per unit cell, an applied field can drive the spin-1/2 chain away from criticality. The explanation for this effect was provided by Affleck and Oshikawa. The alternating coordination environment induces a transverse staggered field and spinon binding. The quantum sine-Gordon model is the relevant low energy field theory and it predicts soliton and breather excitations at specific energies and wave vectors that we compare to the experiments. We shall also compare a complete measurement of the dynamic spin correlation function for CDC in a field to exact diagonalization results for a spin-1/2 chain with a staggered and uniform magnetic field [4]. \newline \newline [1] M. B. Stone, D. H. Reich, C. Broholm, K. Lefmann, C. Rischel, C. P. Landee, and M. M. Turnbull, Phys. Rev. Lett. \textbf{91}, 037205 (2003). \newline [2] M. Kenzelmann, Y. Chien, C. Broholm, D. H. Reich, and Y. Qiu,~ Phys. Rev. Lett. \textbf{93}, 017204 (2004). \newline [3] D. C. Dender, P. R. Hammar, Daniel H. Reich, C. Broholm, and G. Aeppli, Phys. Rev. Lett. \textbf{79}, 1750 (1997). \newline [4] M. Kenzelmann, C. D. Batista, Y. Chen, C. Broholm, D. H. Reich, S. Park, and Y. Qiu, Phys. Rev. B \textbf{71}, 094411 (2005). [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 9:12AM |
U2.00002: Ordering and Excitations in the Field-Induced Magnetic Phase of Cs$_{3}$Cr$_{2}$Br$_{9}$ Invited Speaker: Cs$_{3}$Cr$_{2}$Br$_{9}$ is an interesting example of interacting spin-dimer system. As in other isotropic antiferromagnets such as Haldane or alternating chains and ladders, the ground state in zero field is a total spin singlet separated from the excited triplet by an energy gap. In a magnetic field $H$, a phase transition occurs at a critical field $H_{c1}$, where the gap to the lowest component of the Zeeman-split triplet closes. Above $H_{c1}$, field-induced magnetic order (FIMO) for spin components perpendicular to $H$ is induced by inter-dimer or inter-chain couplings. The FIMO transition may be considered as a Bose-Einstein Condensation. Cs$_{3}$Cr$_{2}$Br$_{9}$ differs from other dimer systems currently studied ($e$.$g$. PHCC, TlCuCl$_{3})$ in two main ways: each Cr$^{3+}$ ion of the dimer has spin 3/2 rather than 1/2 for Cu-based systems and the arrangement of the dimers is hexagonal. This gives rise to anisotropy and frustration in a 3D lattice, respectively. The possibility of studying the magnetic ordering and the spin dynamics in a FIMO with sufficient detail to bring out features of frustration and anisotropy motivated the present neutron scattering study in Cs$_{3}$Cr$_{2}$Br$_{9}$*. Two field orientations have been exploited, perpendicular and parallel to the easy axis \textbf{c} (direction of the dimers). First, I present the diffraction study: the FIMO displays large hysteresis incommensurability, showing the importance of frustration. The impact of anisotropy is seen in the magnetic structure, whose nature strongly depends on the field direction. Second, I focus on spin dynamics: it quantifies the presence of anisotropy and shows its crucial role on the energy gap at $H_{c1}$, which is measurably open or not, depending on whether $H$ is perpendicular or parallel to \textbf{c}. Third, an explanation is proposed for the large value of the gap at higher field: it involves the mixing of higher order states (\textit{extended}-FIMO), reflected by the absence of magnetization plateaus. Comparison with the sister Cs$_{3}$Cr$_{2}$Cl$_{9}$ compound provides a test of this hypothesis. *B. Grenier \textit{et al.,} Phys. Rev. Lett. \textbf{92}, 177202 (2004) [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:48AM |
U2.00003: Quantum phase transitions in integer spin chains Invited Speaker: High field inelastic neutron scattering experiments on the $S$=1 bond-alternating 1D antiferromagnet NTENP, the anisotropic $S=1$-chain Haldane-gap compound NDMAP and the isotropic ``composite'' Haldane spin chain IPA-CuCl$_3$ [T. Masuda {\it et al.}, cond-mat/0506382] reveal key differences in the spin dynamics of these distinct types of quantum spin liquids. In modest applied fields the spectra of NDMAP [A. Zheludev {\it et al.}, Phys. Rev. Lett. {\bf 88}, 077206 (2002)] and IPA-CuCl$_3$ feature three sharp stable gap excitations. In contrast, in NTENP the highest mode is anomalously weak at $H=0$ and rapidly broadens and vanishes when the field is turned on. Above the critical field of 1D Bose condensation of magnons and long-range ordering NDMAP retains a triplet of massive long-lived excitations [A. Zheludev {\it et al.}, Phys. Rev. B $\bf{68}$, 134438 (2003)]. In IPA-CuCl$_3$ only two sharp gap excitations persist, with possibly an additional gapless mode. In NTENP only one sharp excitation branch is observed in this regime [Hagiwara {\it et al.}, Phys. Rev. Lett. {\bf 94}, 177202 (2005)], but there is new evidence of low-lying excitation continua. Work at ORNL was carried out under DOE Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:24AM |
U2.00004: High-field ESR and thermodynamic studies of uniform and bond-alternating $S$=1 spin chains Invited Speaker: Recently, field-induced phenomena in quantum spin systems have attracted considerable interest. Gapped one-dimensional (1D) spin systems with a spin value$ S$=1 subject to an external magnetic field strong enough to close the gap ($H_{c})$ are driven into a new phase. Spin excitations in this field-induced phase have been studied by experiments on a uniform S=1 antiferromagnetic spin chain Ni(C$_{5}$H$_{14}$N$_{2})_{2}$N$_{3}$(PF$_{6})$, alias NDMAP and a bond-alternating one Ni(C$_{9}$H$_{24}$N$_{4})$NO$_{2}$(ClO$_{4})$, alias NTENP. We performed high-field and multi-frequency ESR experiments at 1.5 K on these compounds and observed gapped excitations above $H_{c}$. Two or three excitation modes were observed depending on the field direction in NDMAP and only one excitation in NTENP. These results are consistent with those obtained by inelastic neutron scattering experiments in a magnetic field. Both compounds exhibit the long-range order (LRO) at a magnetic field above $H_{c}$ and a low temperature. Observed gapped excitations are very different from those expected from a conventional spin-wave theory in the LRO state. For NDMAP, observed branches satisfactorily agree with those analyzed by a phenomenological field theory. The difference of observed gapped excitations between NDMAP and NTENP can be explained by an interaction with a low-lying two magnon continuum at q=$\pi $ that is present in a bond-alternating chain but absent in a uniform one. When an antiferromagnetic spin chain with$ S$=1 has an XY or Heisenberg symmetry, the phase above $H_{c}$ is critical and its low-energy physics is described by a Tomonaga-Luttinger liquid (TLL), which is characterized by a gapless $k$-linear energy dispersion with an incommensurate $k_{0}$ and a spin correlation having an algebraic decay. NTENP has nearly an XY symmetry and a linear temperature($T)$ dependence of the specific heat ($C_{mag})$ was observed for the magnetic field parallel to the chain above $H_{c}$ in a temperature region above that of the LRO state. The ratio $C_{mag}$/$T$ increases as the magnetic field approaches $H_{c}$ from above and is in good agreement with the prediction of the $c$=1 conformal field theory, providing a conclusive evidence for a TLL in a gapped quasi-1D antiferromagnet. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 11:00AM |
U2.00005: Anisotropic Haldane-gap chains in a magnetic field Invited Speaker: We consider quasi one dimensional spin-1 Heisenberg chains with crystal field anisotropy in a uniform magnetic field. We determine the dynamical structure factor in various limits and obtain a fairly complete qualitative picture of how it changes with the applied field. In particular, we discuss how the width of the higher energy single magnon modes depends on the field. We consider the effects of a weak interchain coupling. We discuss the relevance of our results for neutron scattering experiments on the quasi-1D Haldane-gap compound NDMAP. [Preview Abstract] |
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