Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L38: Spin Chains, Criticality and QPTFocus

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Sponsoring Units: GMAG DMP Chair: Christopher Landee, Clark University Room: BCEC 206B 
Wednesday, March 6, 2019 11:15AM  11:51AM 
L38.00001: Magneticfield induced quantum phase transitions in spin1/2 XXZ chain materials Invited Speaker: Thomas Lorenz Lowdimensional quantum spin systems offer an ideal playground to study the generic behavior close to magneticfield induced quantum phase transitions. Of particular interest is the XXZ spin1/2 chain model with anisotropy parameter Δ=J_{z}/J_{xy} of the exchange couplings acting either on the z or the x(y)spin components. For Δ=0, 1 and ∞, respectively, the exactly solvable XY, Heisenberg and Ising chain models are covered, but real materials are typically located in between these models and there are additional intra and/or interchain couplings. In this contribution, our recent experimental studies of model materials with fieldinduced quantum phase transitions will be presented. Cu(C_{4}H_{4}N_{2})(NO_{3})_{2}, is an almost ideal realization of the Heisenberg chain with weak intrachain coupling, such that we could study its field induced quantum phase transition in great detail. The experimental data are almost perfectly reproduced by BetheAnsatz calculations over a wide temperature and field range and on approaching the quantum critical field the systematic evolution of the generic quantum critical behavior is clearly observed in the experimental data [1]. In BaCo_{2}V_{2}O_{8}, the Co^{2+} ions realize effective spin1/2 chains with pronounced Ising anisotropy. Here, sizable interchain couplings cause longrange, 3D antiferromagnetic order with complex magneticfield temperature phase diagrams [2]. For a particular transversefield direction, however, the fieldinduced suppression of the 3D magnetic order is well separated from the region where we observe the characteristic quantum critical behavior expected for the onedimensional Ising spin chain in transverse magnetic field [3]. 
Wednesday, March 6, 2019 11:51AM  12:03PM 
L38.00002: Impact of pressure on the magnetic order in an S=1/2 quantum antiferromagnet Tao Hong, Qing Huang, Sachith Dissanayake, Yiming Qiu, Yan Wu, Huibo Cao, Wei Tian, Haidong Zhou, Mark M. Turnbull Here we present a neutron scattering study on a spin1/2 twoleg ladder antiferromagnet C_{9}D_{18}N_{2}CuBr_{4} (DLCB for short) under applied hydrostatic pressure. In DLCB, the interladder coupling is sufficiently strong to drive the system to the Néel antiferromagnetic ordering phase below T_{N}=2 K and the analysis of the spin Hamiltonian reveals that DLCB is close to the quantum critical point in two dimensions at zero field and ambient pressure [1]. Singlecrystal neutron diffraction measurements under pressures up to 1.3 Gpa suggest that size of the staggered moment becomes suppressed with increase of pressure and the magnetic order breaks down above pressure ~1.0 Gpa. 
Wednesday, March 6, 2019 12:03PM  12:15PM 
L38.00003: High pressure and high magnetic field tuning of the S = 1 quantum antiferromagnet system Dan Sun, Sam Curley, Paul Goddard, Jacqueline Villa, Jamie Manson, John Singleton, Fedor Balakirev [Ni(HF_{2})(pyz)_{2}]SbF_{6} is a quasionedimensional S = 1 antiferromagnet in which the magnetic ground state can be tuned by changing the singleion anisotropy (D) and the intrachain (J ) and interchain (J’) exchange interactions, allowing exploration of the Haldane phase diagram close to quantum criticality. With a newly developed pressure cell designed for pulsed magnetic fields of up to 100 T, we tune the above interactions in [Ni(HF_{2})(pyz)_{2}]SbF_{6} using hydrostatic pressures of up to 5.4 GPa. The characteristic form of the magnetic susceptibility was measured in pulsed fields of up to 65 T using a Proximity Detector Oscillator (PDO), allowing values of D, J and J’ to be inferred at each pressure. 
Wednesday, March 6, 2019 12:15PM  12:27PM 
L38.00004: PressureTuning through the D/J ≈ 1 Quantum Critical Point in an S = 1 Antiferromagnetic Chain Marcus K. Peprah, Paul S. Edwards, John Cain, Orlando Trejo, Jaynise M. Perez, Pedro A. Quintero, Jared B Singleton, Larry Paul Engelhardt, Saul H. Lapidus, Erik Cizmar, Jamie Manson, Mark Meisel Properties of [Ni(HF_{2})(3Clpy)_{4}]BF_{4} (py = pyridine), or NBCT, suggest this S = 1 chain is near the D/J ≈ 1 quantum critical point with easyplane anisotropy D = 4.3 K, intrachain antiferromagnetic interaction J = 4.86 K, and no longrange order to 25 mK [1]. To clarify other work [2], the low field (0.1 T) susceptibility χ(2 K < T < 300 K) was studied as a function of pressure (P < 1.5 GPa). These data are contrasted with isothermal (300 K) powder XRD studies made with P < 2.5 GPa. The pressure evolution of χ(T) correlates with structural modifications. With no evidence of a structural transition, the χ(T) data can be simulated by changes of J and D [3]. Complemented by inelastic neutron studies (P and B = 0 and T > 300 mK) [4], NBCT appears to be the first S = 1 antiferromagnetic chain that can be pressuretuned from the Haldane phase to the LargeD regime by traversing the D/J ≈ 1 quantum critical point. 
Wednesday, March 6, 2019 12:27PM  12:39PM 
L38.00005: z=2 quantum critical dynamics in spin chain and spin ladder compounds Dominic Blosser, Vivek Bhartiya, Noam Kestin, Kirill Povarov, David J. Voneshen, Robert Bewley, Emanuele Coira, Thierry Giamarchi, Andrey Zheludev By means of high resolution inelastic neutron scattering, we investigate finite temperature critical dynamics near the magnetic field induced quantum critical point with dynamical exponent z=2 in one dimension. 
Wednesday, March 6, 2019 12:39PM  12:51PM 
L38.00006: The Magnetocaloric Effect in Exotic Spin Chain Compounds Robert Williams, Paul Goddard, Sam Curley, Yoshimitsu Kohama, Akira Matsuo, Sydney Kaech, Zachary Manson, Jamie Manson The seminal S=1/2 chain model is highly sensitive to deviations from the ideal Hamiltonian which, together with the intrinsic quantum fluctuations due to lowdimensionality, can induce a range of exotic behaviours. One such perturbation is an alternating local crystal structure, which produces a fieldinduced gap to solitonic excitations and promotes noncollinear spin structures. The sineGordon (SG) model captures this behaviour at low fields, but breaks down as systems approach saturation. We report the results of pulsedfield adiabatic measurements of the magnetocaloric effect in the archetypal SG chain material [(pym)Cu(NO_{3})_{2}(H_{2}O)_{2}], plus the novel chiral spinchain compound with fourfold periodicity along the chain axis: [Cu(pym)(H_{2}O)_{4}]SiF_{6}.H_{2}O, (pym=N_{2}C_{4}H_{4}). The chiral system displays a rich variety of excitations above a gap which, in contrast to the SG model, has a linear fielddependence and suppressed magnitude. These measurements provide a powerful means of probing both the magnetic entropy across phase diagrams, and the quantum critical behaviour near saturation. Our results highlight similar underlying physics in the compounds, but also indicate intriguing qualitative differences in the quantum phase transitions at high field. 
Wednesday, March 6, 2019 12:51PM  1:03PM 
L38.00007: Pressure induced magnetic order in the novel 1D magnet K_{2}Cr_{8}O_{16} Ola Kenji Forslund, Elisabetta Nocerino, Daniel Andreica, Yasmine Sassa, Hiroshi Nozaki, Izumi Umegaki, Viktor Jonsson, Zurab Guguchia, Zurab Shermadini, Rustem Khasanov, Masahiko Isobe, Yutaka Ueda, Jun Sugiyama, Martin Månsson, Hidenori Takagi The title compound, K_{2}Cr_{8}O_{16}, belongs to a series of quasi1D compounds synthesised using a high pressure/temperature technique. The channels are formed by zigzag Cr_{2}O_{4} chains parallel to the caxis and K cations occupying the center. Intriguingly, the compound undergoes a metal to insulator transition while maintaining the ferromagnetic order [1, 2], established by a Peierls transition [3]. Pressure dependent studies on this compound is fairly limited and the complete phase diagram of this compound is not fully resolved, especially the low temperature / high pressure region [4]. Here, pressure dependent muon spin rotation/relaxation (μSR) data is presented, which uniquely allow us perform measurements in zero applied field and hereby access the true intrinsic material properties. Finally, neutron diffraction has recently been conducted in order to elucidate the detailed nature of the complex phase diagram. 
Wednesday, March 6, 2019 1:03PM  1:15PM 
L38.00008: Magnetic exchange interactions in the BaM_{2}Si_{2}O_{7}(M= Cu, Co, Mn) system Weidong Luo, Chengyang Xu, Guohua Wang, Jie Ma A quasi 1D system with weak exchange coupling between the magnetic chains will exhibit a crossover from 1D magnetic behavior at high temperatures to a 3D ordered state at low temperatures. The BaM_{2}Si_{2}O_{7}(M = Cu, Co, Mn) with layered structure is an excellent system to study lowdimensional magnetic behaviors. BaCu_{2}Si_{2}O_{7} has an orthorhombic structure with CuO_{4} plaquettes, while BaCo_{2}Si_{2}O_{7} and BaMn_{2}Si_{2}O_{7} have a monoclinic crystal structure with CoO_{4} and MnO_{4} tetrahedrons. We have performed local spindensity approximation with onsite Coulomb interaction (LSDA + U) calculations to study the exchange interactions. By computing the total energies of various magnetic configurations and mapping these energies to a Heisenberg model, we extract the magnetic exchange interactions in the three materials. We also discuss the relations between the magnetic exchange interactions with their crystal and electronic structures. In the end, we make comparisons with experimental studies such as single crystal neutron diffraction measurements. 
Wednesday, March 6, 2019 1:15PM  1:27PM 
L38.00009: WITHDRAWN ABSTRACT

Wednesday, March 6, 2019 1:27PM  1:39PM 
L38.00010: Quantized magnonexcitation continuum in quasi onedimensional antiferromagnetic S=1 Heisenberg systems Takafumi Suzuki, Seiichiro Suga In recent inelastic neutron scattering measurements on (Ba/Sr)Co_{2}V_{2}O_{8}[1,2], quantization of spinonexcitation continuum has been observed below the Néel temperature. The interesting point is that the excitation energies of the quantized spectra are well explained by negative zeros of the Airy functions (NZAF)[3]. In this study, we investigate the possibility of quantized magnonexcitation continuum in quasionedimensional antiferromagnetic S=1 Heisenberg chains with the singleion anisotropy, D. We use the infinite timeevolvingblockdecimation method to compute dynamical spin structure factors and show that the magnonexcitation continuum is also quantized by the weak interchain interaction[4]. We find that the excitation energies agree with NZAF when D is quite large negatively. Although the quantization spectra survive up to D ~ 0, the excitation energies deviate from NZAF. [1] Z. Wang, et al., PRB 91, 140404(R) (2015). [2] B. Grenier, et al., PRL 114, 017201 (2015). [3] H. Shiba, Prog. Theor. Phys. 64, 466 (1980). [4] T. Suzuki, and S. Suga, arXiv:1808.06270. 
Wednesday, March 6, 2019 1:39PM  1:51PM 
L38.00011: Resonant secondorder optical resonances in quantum magnets Shunsuke Furuya Recently, laser control of materials has attracted much attention in various fields of condensedmatter physics. In 2016, Sato, Takayoshi, and Oka proposed an ultrafast way to add to a quantum magnet an effective DzyaloshinskiiMoriya interaction dynamically. They confirmed their theoretical proposal by calculating the vector chirality to which the DzyaloshinskiiMoriya interaction is coupled. They also pointed out numerically that there exists a resonance of the dynamically generated vector chirality when the frequency of the applied laser equals to the Zeeman energy of the quantum magnet. This resonance is interesting because the dynamical generation of the vector chirality is the secondorder response to the laser but its resonance resembles another linearresponse phenomenon to the laser, that is, electron spin resonance. Unfortunately, the resonant secondorder response to the laser is yet to be understood. In this presentation, I discuss the origin of the resonant secondorder resonance and exemplify it in quantum spin chain systems. I also show that the resonant secondorder response can be found in quite a general situation of quantum magnets. For example, the magnetization also exhibits the same resonant secondorder optical response. 
Wednesday, March 6, 2019 1:51PM  2:03PM 
L38.00012: Nonordinary Edge Criticality of TwoDimensional Quantum Critical Magnets Lukas Weber, Francesco Parisen Toldin, Stefan Wessel Based on largescale quantum Monte Carlo simulations, we examine the correlations along the edges of twodimensional semiinfinite quantum critical Heisenberg spin1/2 and spin1 systems. In particular, we consider coupled quantum spindimer systems at their bulk quantum critical points, including the columnardimer model and the plaquettesquare lattice. The alignment of the edge spins strongly affects these correlations and the corresponding scaling exponents, with remarkably similar values obtained for various quantum spindimer systems. We furthermore observe subtle effects on the scaling behavior from perturbing the edge spins that exhibit the genuine quantum nature of these edge states. 
Wednesday, March 6, 2019 2:03PM  2:15PM 
L38.00013: Spincurrent diode with a monoaxial chiral magnet Shun Okumura, Hiroaki Ishizuka, Yasuyuki Kato, Junichiro Ohe, Yukitoshi Motome Chiral magnets often show interesting magnetic and transport properties associated with their peculiar spin textures. For instance, a monoaxial chiral magnet CrNb_{3}S_{6} shows a chiral soliton lattice in a magnetic field, accompanied by peculiar temperature and field dependence of resistivity [1]. Such chiral spin textures potentially give rise to nonreciprocal transport phenomena, as they break spatial inversion and time reversal symmetries simultaneously [2]. However, nonreciprocal transport in the monoaxial chiral magnets has not been fully understood, especially for spin currents. Here, we investigate the spindependent transport properties in a chiral conical magnetic state using a onedimensional Kondo lattice model. We calculate the conductance of spin current numerically by using the Landauer method based on Green’s functions. We show that the system exhibits nonreciprocal spin transport, which depends on the conical angle, the chirality of the magnetic structures, and the polarized direction of the spin current. We discuss the origin of the nonreciprocity by analyzing the spin states near the edges. 
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