Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M31: Focus Session: Ladders, Layers, and Dimers |
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Sponsoring Units: GMAG DMP Chair: Jonathan Friedman, Amherst College Room: 207A |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M31.00001: Heat transport in the frustrated spin-ladder compound, BiCu$_2$PO$_6$ Narayan Prasai, Alwyn Rebello, Joshua L. Cohn, Sueli H. Masunaga, John J. Neumeier We report measurements of thermal conductivity ($\kappa$) in the range $0.4\ {\rm K}\leq T\leq 300$~K for single-crystal BiCu$_2$PO$_6$, a recently discovered\footnote{B. Koteswararao {\it et al.}, Phys. Rev. B {\bf 76}, 052402 (2007).} frustrated 2-leg spin-ladder compound. For heat flow both along and transverse to the spin ladders, $\kappa$ exhibits a broad maximum near 60 K, coinciding with a similar maximum reported in the magnetic susceptibility, and consistent with resonant phonon scattering from spin excitations with an energy scale 40-60 K. Anisotropy in $\kappa$, evidence for a spin contribution at low temperatures, and the influence of magnetic field will be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M31.00002: Evolution of magnetic excitations in BiCu$_{2}$PO$_{6}$ observed via magnetic suppression of phonon heat conduction Byung-Gu Jeon, B. Koteswararao, C.B. Park, Kee Hoon Kim, G.J. Shu, F.C. Chou, S.C. Riggs, S.B. Chung We report the thermal conductivity of a frustrated spin ladder BiCu$_{2}$PO$_{6}$ under high magnetic field up to 30 T. At 0 T, strong suppression of the thermal conductivity is emerged around 15 K, leading a double-peak shape in the temperature-dependent thermal conductivity. Upon increasing the magnetic field, the suppression is further enhanced and shows a sharp dip in the magneto-thermal conductivity around the critical magnetic field. The anomalous, field-dependent thermal conductivity is interpreted as a resonance scattering of phononic heat carriers by magnetic excitations. From the analysis based on the transport theory, we successfully traced the magnetic field dependence of the magnetic excitation gaps up to 25 T. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M31.00003: Stepwise orderings in anisotropically coupled spin ladders Shunsuke Furuya, Thierry Giamarchi In low-dimensional quantum magnets, lowering the temperature,
we can find phase transitions from lower to higher dimensional phases.
The (unfrustrated) spin ladder is a good example to see such a change of
the dimensionality because it occurs only under strong enough magnetic
field.
That is, the dimensionality in the coupled spin ladder system is
controllable by means of the temperature and the magnetic field.
In this presentation, we will discuss a cascade of the
dimensionality from the 1D phase to the 3D ordered phase via a
quasi-2D ordered phase and its interesting manifestation
in stepwise temperature dependence of the order parameter.
We presume two inequivalent interladder exchange interactions $J'_1$ and
$J'_2$.
The former connects spin ladders in a plane to form 2D layers and the
latter connects those layers.
For $0 |
Wednesday, March 4, 2015 11:51AM - 12:27PM |
M31.00004: Temporal correlations in tunable Luttinger spin liquids Invited Speaker: Andrey Zheludev Landau's theory of Fermi liquids, which is the cornerstone of our understanding of fermionic systems, breaks down in low dimensions. In one dimension, interacting fermions are in a quantum critical state with some fascinating universal thermodynamic properties and correlation functions. This particularly interesting case is quantitatively described by the so-called Luttinger liquid theory. The cleanest real-world realizations of this model are found in low-dimensional spin systems [1], such as in Heisenberg spin chain and ladder materials. The universal scaling relations in these Luttinger spin liquids can be tested experimentally. In this quest, neutron scattering has proven to be instrumental, as it provides direct access to spatial and temporal correlation functions. In classic previous studies [2,3], this technique has been employed to measuring finite-temperature scaling in the simplest spin chain models. The latter are described by Luttinger liquid theories with the so-called Luttinger parameter K$=$1/2, corresponding to a strong repulsion between particles. The new challenge is to investigate the scaling for other values of K, particularly in systems with K\textgreater 1 (attractive fermions). In experiments, K and other characteristics of Luttinger spin liquids can be, in principle, tuned continuously by the application of an external magnetic field. In practice, measurements under such conditions are extremely challenging due to several unexpected technical difficulties. Nevertheless, recent advances in neutron instrumentation, particularly at pulsed neutron sources, help overcome these obstacles. In my talk I shall review the most recent results of experimental studies of Luttinger liquid properties of low dimensional quantum magnets under high magnetic fields. I will cover spin chain materials where K is continuously tunable in the range 1/2\textless K\textless 1, and the strong leg spin ladder compound DIMPY where K\textgreater 1 was achieved for the first time [4-6]. I will also describe how residual 3-dimensional interactions, usually considered a nuisance for low-dimensional physics, can in certain cases be exploited to accurately measure the exact field dependencies of all relevant one-dimensional Luttinger parameters. \\[4pt] [1] F. D. M. Haldane, Phys. Rev. Lett. \textbf{50}, 1153 (1983).\\[0pt] [2] D. C. Dender, Ph.D. thesis, Johns Hopkins University, 1997.\\[0pt] [3] B. Lake et al., Nature Mater. \textbf{4}, 329 (2005).\\[0pt] [4] D. Schmidiger et al., Phys. Rev. Lett. \textbf{108}, 167201 (2012); Phys. Rev. Lett. \textbf{111}, 107202 (2013).\\[0pt] [5] M. Jeong et al., Phys. Rev. Lett. \textbf{111}, 106404 (2013).\\[0pt] [6] K. Yu. Povarov, et al., arXiv:1406.6876 [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M31.00005: An extremely well-isolated 2D antiferromagnet Christopher Landee, Nizar Naitlho, M.M. Turnbull We report on the synthesis, structure, and magnetic susceptibility of an extremely well-isolated rectangular Heisenberg, S $=$ 1/2 antiferromagnet, [Cu(pz)$_{2}$(4-OHpy)$_{2}$](PF$_{6}$)$_{2}$, where pz $=$ pyrazine and 4-OHpy $=$ 4-pyridone. The copper and pyrazine form nearly square layers of pyrazine-bridged copper(II) ions, with the pyridone molecules normal to the layers, coordinated to the copper sites by the oxygen. The distance between copper sites in adjacent layers is approximately 13 {\AA} indicating a high degree of two-dimensionality. The magnetic susceptibility is best described by a model of an antiferromagnetic rectangle with the stronger and weaker interactions of 8.3 K and 4.4 K, respectively. The compound is more symmetric than an analogous compound [Cu(pz)$_{2}$(2-OHpy)$_{2}$](ClO$_{4}$)$_{2}$, reported previously [1]. \\[4pt] [1] V. Selmani, C. P. Landee, M. M. Turnbull, J. L. Wikaira, and F. Xiao. Inorg. Chem. Comm. 13, 1399-1401 (2010), doi: 10.1016/j.inoche.2010.07.045 [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M31.00006: Dynamics of a quasi-2D S=1/2 spin-dimer Heisenberg antiferromagnet under hydrostatic pressure Gerard Perren, Johannes Moeller, Dan Huevonen, Andrey Podlesnyak, Andrey Zheludev We present Inelastic Neutron Scattering measurements (INS) under hydrostatic pressure on a quasi-2D $S=1/2$ spin-dimer Heisenberg antiferromagnet. At high pressure, the observed dynamic structure factor $S(\vec{Q},\omega)$ features Goldstone modes, which is a key signature of long-range (magnetic) order. This is in good agreement with recent $\mu$SR findings and the presence of a pressure-induced quantum critical point at a moderate pressure [1]. Furthermore, we suggest an explanation for an apparent contradiction with a previous INS study performed under similar conditions [2].\newline \newline [1] M.~Thede \textit{et al.}, \textit{Phys.~Rev.~Lett.} \textbf{112}, 087204 \newline [2] T.~Hong \textit{et al.}, \textit{Phys.~Rev.~B} \textbf{82}, 184424 [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M31.00007: Magnetic phase diagram of the spatially anisotropic spin-$1/2$ zigzag ladder Mohammad Soltanieh-Ha, Adrian Feiguin We study the magnetic phase diagram of a spatially anisotropic zigzag ladder with exchange interactions $J_2$, and $J_2'$ along the legs. This system interpolates between the conventional $J_1$-$J_2$ chain, and the ``sawtooth,'' or ``delta'' chain, described by a linear arrangement of triangles. Both systems display dimerization in a region of the zero-field phase diagram, but the anisotropy leads to two kinds of excitations with different velocities. We study the magnetic phase diagram of the system as a function of $J_2$ and $J_2'$ and find commensurate plateaus at $m=1/2$ and $m=1/3$, and study the competition between different orders, including incommensurate, chiral, polar, and two-component liquid phase. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M31.00008: Symmetry protected topological states in antiferromagnets with magnetic field Shintaro Takayoshi, Keisuke Totsuka, Akihiro Tanaka A symmetry protected topological (SPT) phase is a short-range entangled state that cannot be adiabatically deformed into a direct product state under some symmetry. We show that magnetization plateau states of one-dimensional antiferromagnets in external magnetic field is in an SPT phase when $S-m$ is odd, where $S$ and $m$ represent a spin quantum number and magnetization per site, respectively, if the system respects a bond-center inversion symmetry. We map the antiferromagnets into a field theory of a nonlinear sigma model with a Berry phase term which has a coefficient proportional to the quantity $S-m$. This term appears in the functional form of the ground state wave function and dictates whether or not the system is in the SPT phase. We verify this prediction through numerical calculations of the entanglement spectra and an analysis using a matrix product state representation. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M31.00009: Detailed analysis of critical points in coupled spin dimer systems Dominik Strassel, Peter Kopietz, Sebastian Eggert Spin dimer systems are a promising playground for the detailed study of quantum phase transitions. In many cases it is sufficient to use the magnetic field as the tuning parameter in order to reach interesting non-trivial critical points and observe a crossover from the characteristic scaling near the critical point to the behavior of a finite temperature phase transition. In order to quantitatively demonstrate those effects and inspired by recent experiments we have started large scale quantum Monte Carlo simulations in order to analyze several different physical quantities in spin dimer systems, namely the susceptibility, the magneto-caloric effect, and the spin stiffness. We discuss in detail how the phase transitions (quantum and finite temperature) are manifest in the characteristic scaling behavior near critical points by comparing them with interacting boson theories. For two dimensional systems the predicted logarithmic corrections cannot be observed, however. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M31.00010: Magnetization Plateaux in a frustrated spin ladder Takanori Sugimoto, Michiyasu Mori, Takami Tohyama, Sadamichi Maekawa Recently, successive phase transitions induced by magnetic field have been observed in BiCu$_2$PO$_6$, whose effective spin model, a frustrated two-leg spin ladder, bridges between the frustrated spin chain and the non-frustrated spin ladder with one-half spins. According to theoretical studies, the frustrated spin chain exhibits one third plateau, although no plateau emerges in non-frustrated spin ladders. Therefore, a simple question arises as to whether the plateaux does appear in the frustrated spin ladder, or not. To clarify this question, we calculate the magnetic-field dependence of magnetization by using density-matrix renormalization-group method. In this calculation, we find some plateaux, which appear in neither the frustrated spin chain nor the non-frustrated spin ladder. Our study is useful to analyze experimental data of BiCu$_2$PO$_6$. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M31.00011: Fractional excitations in the square-lattice quantum antiferromagnet H.M. R\O{}nnow, M. Mourigal, B. Dalla Piazza, N.B. Christensen, G.J. Nilsen, T.G. Perring, M. Enderle, D.F. McMorrow, D.A. Ivanov The quantum square-lattice Heisenberg antiferromagnet (QSLHAF) exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wavevector ($\pi$,0). We used polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound Cu(DCOO)$_2\cdot$4D$_2$O (CFTD), a known realization of the QSLHAF model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wave functions [1]. The excitation continuum is accounted for by the existence of pairs of fractional S = 1/2 quasiparticles that deconfine over intermediate length-scales. Away from the anomalous wavevector, these fractional excitations are bound and form conventional magnons. Our results reveal the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration. [1] B. Dalla Piazza {\it et al.}, to appear in Nature Physics (December 2014) [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M31.00012: Generalization of Magnetic Dimer Excitations G. Houchins, J.T. Haraldsen Magnetic dimers commonly appear in the study of molecular magnets and quantum dots. Here, we discuss analytical representations for the inelastic neutron scattering excitation cross sections and static structure factor for the general S$_1$S$_2$ dimeric system. Employing generalized Pauli matrices and the Kronecker tensor product to construct the matrix representation of the spin Heisenberg spin-spin Hamiltonian. After using exact diagonalization to determine the eigenstates of the spin Hamiltonian, we formulated an analytical solution to find the structure factor coefficients used in determining the inelastic neutron scattering excitation cross section from both the ground state and first excited state. We also detail a method for finding the S$_z$ polarization constant within an applied field that may represent the presence of an external magnetic field. Furthermore, we provide a sample set of data and intensity plot generated from our results to illustrate experimental representations for split energy levels. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M31.00013: ABSTRACT WITHDRAWN |
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