Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V34: Focus Session: Frustrated and Low-D Magnetism -- Quantum Magnetism I |
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Sponsoring Units: DMP GMAG Chair: Igor Zaliznyak, Brookhaven National Laboratory Room: E144 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V34.00001: Field-induced Bose-Einstein Condensation of triplons up to 8 K in Sr3Cr2O8 A.A. Aczel, Y. Kohama, C. Marcenat, F. Weickert, M. Jaime, O.E. Ayala-Valenzuela, R.D. McDonald, S.D. Selesnic, H.A. Dabkowska, G.M. Luke Single crystals of the spin dimer system Sr$_3$Cr$_2$O$_8$ have been grown for the first time. Magnetization, heat capacity, and magnetocaloric effect data up to 65 T reveal magnetic order between applied fields of H$_{c1}$ $\sim$ 30.4 T and H$_{c2}$ $\sim$ 62 T. This field-induced order persists up to T$^{max}_c$ $\sim$ 8 K at H $\sim$ 44 T, the highest observed in any quantum magnet where H$_{c2}$ is experimentally-accessible. We fit the temperature-field phase diagram boundary close to H$_{c1}$ using the expression T$_c$ $=$ A(H-H$_{c1}$)$^\nu$. The exponent $\nu$ $=$ 0.65(2), obtained at temperatures much smaller than T$^{max}_c$, is that of the 3D Bose-Einstein condensate (BEC) universality class. This finding strongly suggests that Sr$_3$Cr$_2$O$_8$ is a new realization of a triplon BEC where the universal regimes corresponding to both H$_{c1}$ and H$_{c2}$ are accessible at $^4$He temperatures. [Preview Abstract] |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V34.00002: Thermal Conductivity and the Boson Mass in the Bose-Einstein Condensate compound NiCl$_{2}$-4SC(NH$_{2})_{2}$ V.S. Zapf, M. Jaime, C.D. Batista, K.A. Alhassanieh, P. Sengupta, A. Sologubenko, J. Mydosh, H. Vieyra, N. Oeschler, F. Steglich, A. Paduan-Filho The quantum magnet NiCl$_{2}$-4SC(NH$_{2})_{2}$ has been shown to exhibit Bose-Einstein Condensation (BEC) of the Ni S = 1 spin system. This compound exhibits field-induced XY antiferromagnetism for magnetic fields along the tetragonal c-axis between H$_{c1}$ = 2.1 and H$_{c2}$ = 12.6 T, and the axial symmetry of the spin environment allows us to understand the quantum phase transitions at H$_{c1}$ and H$_{c2}$ in terms of Bose-Einstein condensation (BEC) of spins. Here the tuning parameter for BEC transition is the magnetic field and not the temperature. It turns out that mass of the bosons that condense can be strongly suppressed by quantum fluctuations, and this effect occurs disproportionately at H$_{c1}$. Here we present new thermal conductivity and specific heat data to probe the effect of quantum fluctuations on the boson mass. We compare the experimental results to quantum Monte Carlo simulations and an effective low-energy theory. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V34.00003: Magnetic Susceptibility of a Dirty BEC System Ni$_{0.85}$Cd$_{0.15}$-4SC(NH$_{2})_{2}$ Liang Yin, Jian-Sheng Xia, Neil Sullivan, Vivien Zapf, Armando Paduan-Filho , Rong Yu, Tommaso Roscilde We report measurements of the magnetic susceptibility of a dirty BEC system of magnons: Ni$_{0.85}$Cd$_{0.15}$-4SC(NH$_{2})_{2}$ (The 15{\%}Cd site-diluted DTN) was studied down to 10 mK. Below 100 mK, the critical fields of BEC $H_{c}$ do not obey the conventional 3D universality class $H_{c}(T)$ -- $H_{c}(0) \quad \sim \quad T^{\alpha }$, where $\alpha $ = 1.5. The observed scaling relation of $\alpha $ = 1/1.15 below 100 mK is in agreement with the numerical simulation of Bose glass for this system. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V34.00004: THz-Time Domain Spectroscopy Study of spin-frustrated BaCuSi$_{2}$O$_{6}$ Takahisa Tokumoto, Haidong Zhou, Stephen McGill, Judy Cherian A spin 1/2 dimer compound BaCuSi$_{2}$O$_{6}$ realizes a field-induced Bose-Einstein condensation above 23.2 T with a field-induced AF ordering of triplets. To directly observe the low-energy gap, we have employed the THz-Time Domain Spectroscopy down to 350 mK and up to 10 T. Preliminary results indicate two novel optical transitions around 0.36 (1.49) and 0.5 (2.07) THz (meV) below the reported spin gap of 3.21 meV$^{1}$. Temperature and magnetic field (B//c) dependences of the transitions will be discussed. Reference: 1. S Sebastian et al., Phys. Rev. B 74, 180401R (2006). [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V34.00005: Theory of Magnetic Field-Induced Bose-Einstein Condensation of Triplons in Ba$_3$Cr$_2$O$_8$ Tyler Dodds, Bohm Jung Yang, Yong Baek Kim We report on a theoretical investigation of the spin-1/2-dimer compound Ba$_{3}$Cr$_{2}$O$_8$ in view of the Bose-Einstein condensation (BEC) of triplet excitations under an applied magnetic field. We apply the self-consistent Hartree-Fock-Popov (HFP) approach to a microscopic Hamiltonian, using the realistic triplon dispersion measured in an inelastic neutron scattering experiment. We investigate the temperature range where the BEC picture of magnetic ordering can be applied in this approach. The effective repulsive interaction between triplons is much weaker in Ba$_{3}$Cr$_{2}$O$_8$ than in the canonical spin-dimer compound TlCuCl$_{3}$. Combined with a narrower triplon band, the smaller interaction in Ba$_{3}$Cr$_{2}$O$_8$ leads to a higher density of triplons at the critical point and a larger HFP correction to the critical applied field. Nonetheless, the HFP approach provides a reasonable explanation of the transverse magnetization and specific heat data of Ba$_{3}$Cr$_{2}$O$_8$. [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V34.00006: Dynamics and quasiparticle relaxation in BEC quantum antiferromagnets Sasha Chernyshev, Suhas Gangadharaiah We discuss the relaxation rate of bosonic excitations near the BEC quantum-critical point in 3D at finite temperature, pertinent to a number of spin-gap quantum antiferromagnets in magnetic field. We demonstrate that the bosonic self-energy has a non-trivial frequency behavior that can be observed in spectroscopic experiments. It is shown that the boson-boson scattering channel is essential for a proper understanding of the thermal transport anomalies in these systems. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:48AM |
V34.00007: Lifetime of spin excitations in non-collinear quantum antiferromagnets Invited Speaker: Non-collinearity of spins in the ordered quantum antiferromagnets can be induced by external field or by frustrating interactions. We demonstrate that the spin-wave excitations in a wide class of non-collinear antiferromagnets have finite lifetime even at $T=0$ due to spontaneous two-magnon decays. We present a detailed analysis of the dynamics of the $S=1/2$ Heisenberg antiferromagnet in the triangular lattice in zero field and in the square lattice in high magnetic field. We show that the spectra of these systems are overdamped in most of the Brillouin zone due to such decays. Their spectra also exhibit threshold-like singularities which lead to further enhancement of the decay rates along special contours in the momentum space. Such an enhancement of damping in certain regions of the Brillouin zone can be used in the analysis of the neutron-scattering data to distinguish the decay-induced spin-wave broadening from the other scenarios that can yield broad spectra of spin excitations. We extend our study to the magnetic field-induced BEC phases, common to many quantum antiferromagnets. We demonstrate that the lifetime effects in the spin excitation spectra are crucial for the understanding of the thermal transport anomalies in these systems. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V34.00008: Quasi-two-dimensional spin-1/2 magnetism in $\alpha$-TeVO$_{4}$ Y. Zhao, J.A. Rodriguez, P. Lemmens, H. Berger, C. Broholm $\alpha$-TeVO$_{4}$ is a monoclinic structure where four spin- 1/2 V$^{4+}$ ions per unit cell form chain-like structures extending along the b-axis. Using the new MACS spectrometer at the NIST Center for Neutron Research, we have carried out a comprehensive investigation of magnetism in this compound. The phase transition at T$_C$ = 16 K is to long range order that can be described as ferromagnetic planes stacked antiferromagnetically along the (1,0,$\bar{2}$) direction. Powder diffraction indicates moments oriented along b-axis with a magnitude of 0.81(3) $\mu_B$ at T=1.4 K. The inelastic scattering data reveal a strongly anisotropic magnon dispersion relation which is almost independent of wave vector transfer along (1,0,$\bar{2}$) at high energies. Work at JHU is supported by the DoE through DE-FG02-08ER46544. [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V34.00009: Magnetic Properties of the quasi-2D S=1/2 Heisenberg antiferromagnet [Cu(pyz)$_2$(HF$_2$)]PF$_6$ Sergei Zvyagin, E. \v{C}i\v{z}m\'{a}r, R. Beyer, M. Uhlarz, M. Ozerov, Y. Skourski, J. Wosnitza, J.L. Manson, J.A. Schlueter We report on electron spin resonance, high-field magnetization, and specific-heat studies of [Cu(pyz)$_2$(HF$_2$)]PF$_6$ single crystals, identified as a quasi-two-dimensional spin-1/2 Heisenberg antiferromagnet. Our measurements revealed $J_{inter}/J_{intra}\leq $ 0.063 and $A/J\sim$ 0.003, where $J_{inter}$, $J_{intra}$, $J$ are the interplane, intraplane and mean exchange interactions, respectively, and $A$ is the anisotropy constant. It is argued that the magnetic properties of this material (including high-magnetic-field magnetization and the temperature-field phase diagram) are strongly affected by two-dimensional spin fluctuations, despite of onset of 3D long-range magnetic ordering at $T_N\approx 4.4$~K. The ESR magnetic excitation spectrum in the 3D ordered phase is studied in detail. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V34.00010: High Field Calorimetry Study on Magnetic Phase Diagram of Quasi-2D QHAF compound Cu(pz)$_2$(ClO$_4$)$_2$ Nathanael Fortune, Scott Hannahs, Fax Xiao, Christopher Landee, Mark Turnbull The ordering temperature of the quasi-2D quantum Heisenberg antiferromagnetic compound Cu(pz)$_2$(ClO$_4$)$_2$ has been found to be strongly influenced by the external field [1]. The ordering temperature of a single crystal of Cu(pz)$_2$(ClO$_4$)$_2$ (less than 1~mg) was determined by calorimetry measurement in applied DC field up to 33T at NHMFL. The magnetic phase diagram shows a round maximum at 18 T (about 1/3 of its saturation field) at which the ordering temperature was raised by 30\%. The experimental result is consistent with a recent prediction for the ideal 2D QHAF system [2], indicating a field induced Heisenberg to XY crossover behavior. Similar crossover behavior has recently been observed between uniaxial and isotropic ordering in a related compound [3]. [1] N. Tsyrulin, F. Xiao et al, submitted to {\it Phys. Rev. B},2009. [2] A. Cuccoli, T. Roscilde, R. Vaia, and P. Verrucchi, {\it Phys. Rev. B}, {\bf 68}, 060402 (2003). [3]P. Sengupta et al, {\it Phys. Rev. B}, {\bf 79},060409(R) (2009). [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V34.00011: Study of the Quantum Phase Transition in the Two-dimensional Heisenberg Antiferromagnet Cr(dien)(O$_2$)$_2$$\cdot$H$_2$O by means of Proton NMR Tiglet Besara, Y.H. Kim, N. Kaur, P.L. Kuhns, A.P. Reyes, Y. Takano, N.S. Dalal We have investigated Cr(dien)(O$_2$)$_2$$\cdot$H$_2$O, a Cr(IV) based compound, by means of proton solid state NMR. The compound is a two-dimensional square-lattice antiferromagnet, experiencing a phase transition at 2.55 K in zero field. We present data from spectrum and spin-lattice relaxation time measurements in the vicinity of the critical field, $H_{c}\approx$12.4 T, and in the temperature region $0.3\--1.5$ K, along with high temperature single crystal angular variation spectra. The spin-lattice relaxation time goes through a minimum as the phase boundary is crossed, emphasizing the change in the spin dynamics of Cr(dien)(O$_2$)$_2$$\cdot$H$_2$O due to fluctuations near the quantum critical point. [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V34.00012: Absence of spin liquid behavior: Magneto-optical study of Nd$_3$Ga$_5$SiO$_{14}$ T.V. Brinzari, X.S. Xu, J.L. Musfeldt, S. McGill, H.D. Zhou, C.R. Wiebe We measured the low-lying crystal field levels of Nd$^{3+}$ in Nd$_{3}$Ga$% _{5}$SiO$_{14}$ via magneto-optical spectroscopy and employed the extracted crystal field energies, magnetic moments, and symmetries to analyze the magnetic properties and test the spin liquid candidacy of this material. The exchange interaction is surprisingly small, a discovery that places severe constraints on models used to describe the ground state of this system. Further, it demonstrates the value of local-probe photo-physical techniques for rare-earth-containing materials where bulk property measurements can be skewed by low-lying electronic structure. [Preview Abstract] |
Thursday, March 18, 2010 10:48AM - 11:00AM |
V34.00013: Frustrated magnetism in doped quasi-triangular lattice materials, Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$/(C$_{7}$H$_{15}$COO) Jian Wu, Fletcher Werner, Anup K. Gangopadhyay, S.A. Solin We have performed DC and AC magnetic susceptibility measurements on the spin S=1/2 quasi-triangular lattice materials Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$/(C$_{7}$H$_{15}$COO). The X-ray diffraction experiments reveal that this class of materials has a crystal structure in P2$_{1}$/m space group, in which Cu$^{2+}$ and Zn$^{2+}$ ions are arranged on a slightly distorted triangular lattice [1]. Cu$_{2(1-)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$ with a short inorganic intercalation NO$_{3}$ group, have a long-range antiferromagnetic order at low temperature. The Neel temperature T$_{N}$ decreases from 11K to 5.6K while the Curie-Weiss temperature increases from -5.1K to +2.8K as the Zn concentration increases from 0 to 65\%. After a longer alkanecarboxylate C$_{7}$H$_{15}$COO group was introduced into the interlayer space, a spin-glass like behavior in magnetic properties was observed [2]. The value $|\Theta_{cw}$ /T$_{N}|$ is approximately 20, indicating the materials are in a medium level frustrated state. The onset of a ferromagnetic correlation was found in both DC and AC susceptibility data. The interplay of geometrical frustration and the coexistence of ferromagnetic and antiferromagnetic couplings has driven the materials into a glassy ground state. \newline [1] G. Linder, et al., Journal of Solid State Chemistry (1995) \newline [2] M. A. Girtu et al, Phys Rev B 61,4117(2000). [Preview Abstract] |
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