Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H19: Spin Chains: TheoryFocus

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Sponsoring Units: GMAG DMP Chair: Igor Zaliznyak, Brookhaven National Laboratory Room: LACC 308A 
Tuesday, March 6, 2018 2:30PM  2:42PM 
H19.00001: Quantum phase transitions and stable edge magnetism in effective spin models for graphene zigzag nanoribbons Stefan Wessel, Cornelie Gergs We derive and examine effective quantum spin models that allow to analyse the interactioninduced magnetic correlations among the edge states of graphene nanoribbons beyond previous meanfield theory approaches, fully accounting for quantum fluctuations. We find that the extended nature of the intraedge couplings in the effective spin model for zigzag nanoribbons leads to a quantum phase transition at a large, finite value of the interedge coupling. This quantum critical point separates a quantum disordered region from a gapless phase of stable edge magnetism at weak intraedge coupling, which includes the ground states of spinladder models for sufficiently wide zigzag nanoribbons. We find that narrow zigzag ribbons of only a few zigzag lines instead reside within the quantum disordered regime. To study the quantum critical behavior, the effective spin model can be related to a model of two antiferromagnetically coupled HaldaneShastry spinhalf chains with longranged ferromagnetic intrachain couplings. The results for the critical exponents compare well to recent renormalization group calculations for related longranged interacting quantum systems. 
Tuesday, March 6, 2018 2:42PM  2:54PM 
H19.00002: TwoTerminal Transport of Quantum Spin Superfluidity Silas Hoffman, Daniel Loss, Yaroslav Tserkovnyak We theoretically study the spin current between two quantum spin chains with easy plane anisotropy separated by an intermediate quantum spin chain with easy axis anisotropy in the same plane. Upon increasing the magnetic field, the middle section undergoes a quantum phase transition from a doubly degenerate ordered phase to a nondenerate quantum paramagnet. In the nongenerate phase, we find that zero energy magnons are exponentially suppressed by the middle section and perfectly reflected when the middle section is sufficiently long. In the degenerate phase, the tunneling is exponentially suppressed until a resonant length is reached in which there is a peak in the transmission of magnons which facilitates transfer of a spin superfluid. We identify a twist in the magnetic order connecting the two degenerate ground states in the middle region as the mechanism responsible for the enhanced tunneling at this resonant length beyond which incident magnons are perfectly reflected with opposite spin. 
Tuesday, March 6, 2018 2:54PM  3:06PM 
H19.00003: Spinon deconfinement on domain walls Tin Sulejmanpasic, Hui Shao, Anders Sandvik A valencebond solid phase of a squarelattice antiferromagnet supports four dimer patterns (vacua) related by the Z4 lattice rotational symmetries. Such a system allows for stable domain walls between regions in different vacua. We here discuss a system in a widestrip geometry, where boundary conditions enforce two parallel domain walls along the long direction. In this system a spin1 excitation deconfines into two spin1/2 spinons, even though such excitations are confined into conventional spin1 "triplons" in the bulk because of the confining strings formed when spinons are separated. In the geometry we consider, the domain walls absorb the confining strings, leading to a finite energy of creating the spinons. We confirm this scenario by quantum Monte Carlo simulations of the JQ spin model, showing that, by tuning the width of the strip (thedistance between the domain walls), we can control the mass of such deconfined spinon excitation. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H19.00004: Nontrivial phases at intermediate rung coupling in SU(N) Heisenberg ladders Andreas Weichselbaum, Sylvain Capponi, Andreas Laeuchli, Philippe Lecheminant, Alexei Tsvelik Ladder models represent a first step away from purely 1D chains into the 2D realm. Here we analyze the Hubbard ladder of N symmetric flavors in the Mott regime with one particle / site, equivalent to the SU(N) Heisenberg ladder with the defining representation at each site. We analyze the latter for N<6 using the density matrix renormalization group (DMRG) in the regime of antiferromagnetic leg and rung coupling which is also most natural for quantum simulatons in ultracold atoms. While the asymmptotic regimes of strong or weak rung coupling J_{⊥} are mostly understood, the full phase diagram for arbitrary J_{⊥}>0 is largely unexplored for N>2. While for even N≤6, the phase diagram turns out rather plain with a single phase for J_{⊥}>0, we find nontrivial intermediate phases for odd N, including incommensuration for N=3 and an extremely narrow phase for N=5 reminiscent of topological phases. By inheriting the exponential numerical complexity of multiorbital models, superior technical tools are required. By focusing on flavorsymmetric models, we fully exploit the underlying SU(N) symmetry in our DMRG simulations based on the QSpace tensor library. This allows us to keep beyond a million of states for larger N which is about two orders of magnitude beyond the current state of the art. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H19.00005: The dynamic structure factor in impuritydoped spin chains Kevin Jägering, Annabelle Bohrdt, Imke Schneider, Sebastian Eggert The effects of impurities in spin1/2 Heisenberg chains are recently experiencing a renewed interest due to experimental realizations in solid state systems and ultracold gases. The impurities effectively cut the chains into finite segments with a discrete spectrum and characteristic correlations, which have a distinct effect on the dynamic structure factor. Using bosonization and the numerical Density Matrix Renormalization Group we provide detailed quantitative predictions for the momentum and energy resolved structure factor in doped systems. Due to the impurities, spectral weight is shifted away from the antiferromagnetic wavevector k=π into regions which normally have no spectral weight in the thermodynamic limit. The effect can be quantitatively described in terms of scaling functions, which are derived from a recurrence relation based on bosonization. We present lengthaveraged and kintegrated results in terms of the doping concentration. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H19.00006: Novel magnetization plateaus in the spin1/2 antiferromagnetic Heisenberg model on a kagomestrip chain Katsuhiro Morita, Takanori Sugimoto, Shigetoshi Sota, Takami Tohyama Quantum phase transitions are one of hot topics in the study of condensed matter physics. In geometrically frustrated quantum spin systems, at zero temperature, quantum phase transitions, such as magnetization plateaus, cusps, and jumps, are frequently induced by applying magnetic field. We focus on a 1D quantum spin frustrated systems, the spin1/2 Heisenberg model on a kagomestrip chain. We study the magnetization plateaus of this model with three antiferromagnetic exchange interactions by the densitymatrix renormalization group method. In a certain range of exchange parameters, we find twelve kinds of magnetization plateaus, nine of which have magnetic structures breaking either translational and/or refection symmetry spontaneously. In a most nontrivial plateau, 3/10 plateau, we find longperiod magnetic structure with a period of four unit cells [1]. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H19.00007: NMR relaxation rate in quasionedimensional antiferromagnets Maxime Dupont, Sylvain Capponi, Nicolas Laflorencie, Edmond Orignac Quasionedimensional antiferromagnets are good candidates to realize a TomonagaLuttinger liquid (TLL), describing 1D interacting quantum systems, provided 3D coupling remains small as compared to temperature. In such systems, made of weakly coupled spin chains, interchain coupling plays an important role at low temperature as it leads to a spontaneous symmetry breaking below the critical temperature. Using timedependent numerical simulations at finite temperature [1] we have computed and studied the NMR relaxation rate in S=1/2 antiferromagnetic XXZ spin chains to address the question of the lowtemperature crossover between TLL predictions and the highertemperature regime. To understand the effect of the threedimensional coupling on the NMR relaxation rate (close to and at the transition), we analyze the full 3D microscopic model using quantum MonteCarlo techniques. This allows to compute dynamical correlations in imaginary time and we will discuss recent advances to perform stochastic analytic continuation to get real frequency spectra that can be compared to analytical selfconsistent approximations results [2]. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H19.00008: Thermal IntraBand Magnon Scattering in Haldane SpinOne Chains Jonas Becker, Thomas Koehler, Alexander Tiegel, Salvatore Manmana, Stefan Wessel, Andreas Honecker We present results from a thorough evaluation of the dynamical spin structure factor of the Haldane chain at finite temperatures, 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H19.00009: Emergent SU(N) symmetry in disordered SO(N) magnetic chains Pedro Lopes, Victor Quito, Jose Hoyos, Eduardo Miranda We show that strongly disordered magnetic chains invariant under the SO(N) group display antiferromagnetic 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H19.00010: A tunable quantum spin chain with threebody interactions Khagendra Adhikari, Kevin Beach We present results for a spin chain model that interpolates between the conventional quantum Heisenberg spin chain (twobody interaction) in the limit of ferromagnetic and antiferromagnetic coupling. In this model, there is a special tuning point at which the model corresponds to the recently introduced, exactly solvable Fredkin spin chain (with its threebody interaction). We solve for the lowenergy properties, numerically and semianalytically, in order to track the properties of the system as it is tuned between the extreme limits and compare our results with the known values at the Fredkin point. We identify a zerotemperature phase diagram with multiple transitions and unexpected ordered phases. 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H19.00011: Disorderinduced Revival of the BoseEinstein Condensation at High Magnetic Fields in Ni(Cl$_{1x}$Br$_x$)$_2$4SC(NH$_2$) Sylvain Capponi, Maxime Dupont, Nicolas Laflorencie, Mladen Horvatić We theoretically investigate the high magnetic field regime of the disordered quasionedimensional S=1 antiferromagnetic material Ni(Cl$_{1x}$Br$_x$)$_2$4SC(NH$_2$). This compound was believed to provide the first experimental realization of the elusive Boseglass phase in a quantum magnet [1]. I will review our recent experimental and theoretical works [25] in which we discovered that this manybody localized phase is actually undermined by the resurgence of longrange order with a condensation of the underlying impurities degree of freedom, leading to a novel “orderbydisorder” mechanism. 
Tuesday, March 6, 2018 4:42PM  4:54PM 
H19.00012: Exposing fractional spin excitations with spin current noise in onedimensional quantum magnets Joshua Aftergood, So Takei We theoretically study the spin current and its dc noise in the bulk of a XXZ spin1/2 antiferromagnetic chain downstream of a weak potential barrier. The system is perturbed by the potential at a single point, thermalized throughout to a finite temperature, and driven at one end by a spin Hallfacilitated injection of spin excitations. We examine the spin Fano factor, defined as the spin current noisetosignal ratio, for evidence of the fractional nature of the spin excitation hosted by the chain, and propose noninvasive methods of generating the potential and of measuring the bulk spin current and noise. 
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