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: Magnetic-field induced quantum phase transitions in spin-1/2 XXZ chain materials Invited Speaker: Thomas Lorenz Low-dimensional quantum spin systems offer an ideal playground to study the generic behavior close to magnetic-field induced quantum phase transitions. Of particular interest is the XXZ spin-1/2 chain model with anisotropy parameter Δ=Jz/Jxy 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 inter-chain couplings. In this contribution, our recent experimental studies of model materials with field-induced quantum phase transitions will be presented. Cu(C4H4N2)(NO3)2, is an almost ideal realization of the Heisenberg chain with weak intra-chain coupling, such that we could study its field induced quantum phase transition in great detail. The experimental data are almost perfectly reproduced by Bethe-Ansatz 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 BaCo2V2O8, the Co2+ ions realize effective spin-1/2 chains with pronounced Ising anisotropy. Here, sizable inter-chain couplings cause long-range, 3D antiferromagnetic order with complex magnetic-field temperature phase diagrams [2]. For a particular transverse-field direction, however, the field-induced suppression of the 3D magnetic order is well separated from the region where we observe the characteristic quantum critical behavior expected for the one-dimensional 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 spin-1/2 two-leg ladder antiferromagnet C9D18N2CuBr4 (DLCB for short) under applied hydrostatic pressure. In DLCB, the inter-ladder coupling is sufficiently strong to drive the system to the Néel antiferromagnetic ordering phase below TN=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]. Single-crystal 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(HF2)(pyz)2]SbF6 is a quasi-one-dimensional S = 1 antiferromagnet in which the magnetic ground state can be tuned by changing the single-ion 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(HF2)(pyz)2]SbF6 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: Pressure-Tuning 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(HF2)(3-Clpy)4]BF4 (py = pyridine), or NBCT, suggest this S = 1 chain is near the D/J ≈ 1 quantum critical point with easy-plane anisotropy D = 4.3 K, intrachain antiferromagnetic interaction J = 4.86 K, and no long-range 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 pressure-tuned from the Haldane phase to the Large-D 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 low-dimensionality, can induce a range of exotic behaviours. One such perturbation is an alternating local crystal structure, which produces a field-induced gap to solitonic excitations and promotes non-collinear spin structures. The sine-Gordon (SG) model captures this behaviour at low fields, but breaks down as systems approach saturation. We report the results of pulsed-field adiabatic measurements of the magnetocaloric effect in the archetypal SG chain material [(pym)-Cu(NO3)2(H2O)2], plus the novel chiral spin-chain compound with four-fold periodicity along the chain axis: [Cu(pym)(H2O)4]SiF6.H2O, (pym=N2C4H4). The chiral system displays a rich variety of excitations above a gap which, in contrast to the SG model, has a linear field-dependence 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 K2Cr8O16 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, K2Cr8O16, belongs to a series of quasi-1D compounds synthesised using a high- pressure/-temperature technique. The channels are formed by zig-zag Cr2O4 chains parallel to the c-axis 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 BaM2Si2O7(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 BaM2Si2O7(M = Cu, Co, Mn) with layered structure is an excellent system to study low-dimensional magnetic behaviors. BaCu2Si2O7 has an orthorhombic structure with CuO4 plaquettes, while BaCo2Si2O7 and BaMn2Si2O7 have a monoclinic crystal structure with CoO4 and MnO4 tetrahedrons. We have performed local spin-density 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
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Wednesday, March 6, 2019 1:27PM - 1:39PM |
L38.00010: Quantized magnon-excitation continuum in quasi one-dimensional antiferromagnetic S=1 Heisenberg systems Takafumi Suzuki, Sei-ichiro Suga In recent inelastic neutron scattering measurements on (Ba/Sr)Co2V2O8[1,2], quantization of spinon-excitation 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 magnon-excitation continuum in quasi-one-dimensional antiferromagnetic S=1 Heisenberg chains with the single-ion anisotropy, D. We use the infinite time-evolving-block-decimation method to compute dynamical spin structure factors and show that the magnon-excitation continuum is also quantized by the weak inter-chain 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 second-order optical resonances in quantum magnets Shunsuke Furuya Recently, laser control of materials has attracted much attention in various fields of condensed-matter physics. In 2016, Sato, Takayoshi, and Oka proposed an ultrafast way to add to a quantum magnet an effective Dzyaloshinskii-Moriya interaction dynamically. They confirmed their theoretical proposal by calculating the vector chirality to which the Dzyaloshinskii-Moriya 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 second-order response to the laser but its resonance resembles another linear-response phenomenon to the laser, that is, electron spin resonance. Unfortunately, the resonant second-order response to the laser is yet to be understood. In this presentation, I discuss the origin of the resonant second-order resonance and exemplify it in quantum spin chain systems. I also show that the resonant second-order response can be found in quite a general situation of quantum magnets. For example, the magnetization also exhibits the same resonant second-order optical response. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L38.00012: Nonordinary Edge Criticality of Two-Dimensional Quantum Critical Magnets Lukas Weber, Francesco Parisen Toldin, Stefan Wessel Based on large-scale quantum Monte Carlo simulations, we examine the correlations along the edges of two-dimensional semi-infinite quantum critical Heisenberg spin-1/2 and spin-1 systems. In particular, we consider coupled quantum spin-dimer systems at their bulk quantum critical points, including the columnar-dimer model and the plaquette-square lattice. The alignment of the edge spins strongly affects these correlations and the corresponding scaling exponents, with remarkably similar values obtained for various quantum spin-dimer 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: Spin-current 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 CrNb3S6 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 spin-dependent transport properties in a chiral conical magnetic state using a one-dimensional 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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