Session A21: Magnetic Layers

Specific Heat Studies of a 2D S $=$ \textonehalf Heisenberg Antiferromagnet

We report on the field-dependent specific heat of a highly two-dimensional Heisenberg, S $=$ 1/2 antiferromagnet (2D QHAF), [Cu(pz)$_{\mathrm{2}}$(2-OHpy)$_{\mathrm{2}}$](ClO$_{\mathrm{4}})_{\mathrm{2}}$, where pz $=$ pyrazine and 2-OHpy $=$ 2-pyridone. The copper atoms and pyrazine molecules form distorted rectangular layers of pyrazine-bridged copper(II) ions with the pyridone molecules normal to the layers, providing exceptional spacing between layers [1]. The zero-field specific heat of this compound (1.8 -- 35 K) is compared to the recent QMC simulations of the specific heat for the 2D QHAF. Under applied field, the temperature dependence of the specific heat varies smoothly, but no field-induced ordering is observed. This behavior differs from the field-induced ordering in the 2D QHAF Cu(pz)$_{\mathrm{2}}$(ClO$_{\mathrm{4}})_{\mathrm{2}}$ reported previously [2]. [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 [2] N. Tsyrulin, F. Xiao, A. P. Schneidewindm H. M. R\"{o}nnow, J. Gavilano, C. P. Landee, M. M. Turnbull, M. Kenzelmann. Phys. Rev. B, 81, 134409 (2010), doi: 10.1103/PhysRevB.81.134409.   

Thickness- and magnetic-field-driven suppression of antiferromagnetism in V$_{5}$S$_{8}$ single crystals

The search for novel materials approaching the $2d$ limit can be expanded beyond the transition metal dichalcogenides (TMDs) to related compounds, widening the range of available physical phenomena and tuning parameters. V$_{5}$S$_{8}$, a metal with an antiferromagnetic (AFM) ground state below $\sim$ 32 K, displays a prominent spin-flop transition at $\sim$ 4.2 T. Here we study the AFM state in thin CVD-grown single crystals of V$_{5}$S$_{8}$, focusing on temperatures close to T$_{N\'eel}$, where the exact transition temperature depends on the crystal thickness. Magnetoresistance (MR) measurements performed just below T$_{N\'eel}$ reveal magnetic hysteresis, likely a result of a first-order magnetic field-driven breakdown of the AFM state. In thin crystals, on the order of 10 nm thick, monotonic MR measurements suggest that antiferromagnetism is suppressed as the thickness nears the $2d$ limit. This work demonstrates the possibility of growing single crystals of a relatively complicated magnetic system with thicknesses approaching one unit cell, thereby allowing the tuning of magnetic properties by a field-driven phase transition.   

Field induced phase transition in layered honeycomb spin system $\alpha$-$\textrm{RuCl}_3$ studied by thermal conductivity

$\alpha$-$\textrm{RuCl}_3$, a quasi –two-dimensional honeycomb lattice is known to be a candidate material to realize the Heisenberg-Kitaev spin model of a highly anisotropic bond-dependent exchange interaction. We investigate in-plane thermal conductivity ($\kappa$) as a function of temperature ($T$) and in-plane applied field ($H$). At $H=0$, the onset of a strong increase in $\kappa$ marks the spontaneous long range ordering temperature, $T_c = 6.5\mathrm{K}$, corresponding to “zigzag” antiferromagnetic ordering. A broad peak appearing below $T_c$ in $\kappa$ was found to be suppressed significantly as $H$ increases up to $\approx 7 \mathrm{T}$, implying the system undergoes a field-induced transition from ordered to a new spin-disordered state analogous to the transverse-field Ising model. Further increasing $H$ above $7.1\mathrm{T}$, the large field seems to begin polarizing spins thus increasing the phonon mean free path, resulting in a significant rise in $\kappa$. This tendency is clearly shown in the field dependence of $\kappa$ below $T_c$, which has a pronounced minimum at $H_{{\mathrm{min}}}=7.1\mathrm{T}$. We will discuss our scaling analysis to characterize this field-induced phase transition and compare to the transverse-field Ising spin system.   

Fingerprints of the field-induced Berezinskii-Kosterlitz-Thouless transition in quasi-two-dimensional quantum magnets

The two-dimensional (2d) easy-plane (XY) model provides a prototypical description of 2d systems exhibiting topological excitations, which drive the Berezinskii-Kosterlitz-Thouless (BKT) transition that occurs in 2d superfluids, electron plasmas, Josephson junction arrays, ultracold atomic 2d Bose gasses, etc. The excitations in the 2d XY model are spin waves and vortices. In the BKT scenario, at low temperatures, all vortices (V) and antivortices (AV) are bound to V-AV pairs, and spin waves dominate in this quasi-long-range-ordered phase with an infinite correlation length, $\xi $, and an algebraic decay of correlations. At a critical temperature, $T_{\mathrm{BKT}}$, the V-AV pairs start to unbind, driving the transition to a free vortex phase above $T_{\mathrm{BKT}}$, characterized by an exponential divergence of $\xi $ [1]$_{\mathrm{.}}$ Vortices remain stable also in quantum 2d anisotropic Heisenberg systems with a very weak XY anisotropy [2]. The BKT scenario appears even in 2d isotropic Heisenberg magnets due to frustration [3] or an external magnetic field [4]. I will focus on quasi-2d spin 1/2 Heisenberg antiferromagnets with extremely weak spin anisotropy [5]. These highly anisotropic layered Cu(II) organo-metallic insulators with relatively low saturation fields, about 6 T, enabled a comprehensive study in a wide range of magnetic fields and temperatures. A response of all compounds to the application of a magnetic field mimics 2d behavior with fingerprints of a field-induced Berezinskii-Kosterlitz-Thouless phase transition. [1] J. V. Jos\'{e} (Ed.), 40 Years of Berezinskii--Kosterlitz--Thouless Theory, World Scientific, 2013. [2] A. Cuccoli et al., Phys. Rev. Lett. 90 (2003) 167205. [3] B. Jeevanesan et al., Phys. Rev. Lett. 115 (2015) 177201. [4] A. Cuccoli et al., Phys. Rev. B 68 (2003) 060402(R). [5] R. Tarasenko et al., Phys. Rev. B 87 (2013) 174401.   

Magnetic Property Determination of Nickel Niobate (NiNb2O6)

We have synthesized a novel polymorph of the material nickel niobate, NiNb2O6, in a previously undetermined space group. We have examined the material using SQUID magnetometry and have observed a magnetic transition at approximately 14 K, and a second magnetic feature below 2 K. We have determined these materials using muon spin rotation and relaxation at TRIUMF National Lab in Vancouver, Canada and using neutron scattering on the HB-3A beamline of the High Flux Isotope Reactor at Oak Ridge National Labs, TN. Using these techniques we were able to determine that the magnetic structure is highly two-dimensional. This talk will discuss the nature of the phase transition and its evolution through low temperatures.   

Direct Visualization of Surface Phase of Oxygen Molecules Physisorbed on the Ag(111) Surface: A Two-dimensional Quantum Spin System

Oxygen molecule (O$_{\mathrm{2}})$ is one of the smallest molecular magnets with an $S=$1 quantum spin. This makes O$_{\mathrm{2}}$ attractive as a building block of low-dimensional (LD) quantum spin systems. Recently, the existence of a spin in physisorbed O$_{\mathrm{2}}$ on Ag(111) was confirmed by the ortho-para conversion of molecular hydrogen. Therefore, there is a strong need for STM-based techniques with single-molecule resolution in order to verify the potential of the O$_{\mathrm{2}}$/Ag(111) for LD quantum spin systems. Here we report the real-space observation of oxygen molecules physisorbed on an Ag(111) surface by using low-temperature scanning tunneling microscopy and spectroscopy. A well-ordered O$_{\mathrm{2}}$ structure was observed, and the lattice was distorted from an isosceles triangular lattice. The distortion can be explained by the competition between the magnetic and elastic instabilities of the O$_{\mathrm{2}}$ lattice. In differential tunneling conductance spectra, we found no feature of the Kondo resonance at 4.7 K; in contrast, the physisorbed O$_{\mathrm{2}}$ on Ag(110) showed a clear Kondo resonance at 18 K. Based on these observations, we discuss the realization of an $S=$1 two-dimensional antiferromagnetic quantum spin system.   

Extent of the Z2 topological phase in the quantum dimer model

The possibility that topological order is realized in the anti-ferromagnetic Heisenberg model on the Kagome lattice has been supported by increasing numerical evidence over the last years. In particular, effective low-energy descriptions in terms of quantum dimers models provide valuable insights. It has been shown that the phase diagram of the quantum dimer model contains a point at which the Hamiltonian is exactly solvable and realizes a Z2 topological phase. We study the extent of this phase around the exactly solvable point. Therefore we consider the low-energy spectrum which we determine by means of high-order perturbation theory.   

Effect of hydrogenation on magnetic properties of heavy transition-metal dichalcogenides

Two-dimensional transition-metal dichalcogenides (2D TMDs) are emerging as a unique class of materials because of their underlying fundamental physics and technological applications in electronics, sensors, energy storage, photonics, and spintronics. The outstanding electronic properties of 2D TMDs can be further tuned by various external means, such as control of external electric field, chemical functionalization, alloying, and strain. The electronic and magnetic properties of chemical functionalized 2D TMDs is of special interest. Experimentally, adsorbed fluorine has been shown very recently to create a small magnetic moment of 0.06 emu/g in MoS$_{2}$ nanosheets. Although several studies as well as review articles on the properties of TMDs have been published in the past few years, Mo and W chalcogenides are most widely studied among the ``beyond-graphene'' 2D TMDs. However, studies of chemical functionalization on TMDs containing heavy TMDs such as Ta and Pt are still infancy. In the present work, we investigate the effect of hydrogenation on the magnetism of PtSe$_{2}$ monolayers using density-functional theory. We find that the hydrogen induces a magnetic moment of 0.7 \textmu $_{B}$ per unitcell. This work has been supported by ARO, and DOE-BES.   

Calculating small interchain exchange parameters in Copper Pyrazine Dinitrate

The coordination polymer copper pyrazine dinitrate (Cu(pyz)(NO3)2) is a one-dimensional antiferromagnet that undergoes a magnetic phase transition to a state of long-range three dimensional magnetic order (LRO) below a temperature of 110 mK. The precise nature of the LRO is dependent on the strength of interchain interactions, which are very weak compared to the dominant superexchange interaction along the chain. It is therefore possible that different approaches to ab initio calculations of exchange interaction parameters may be subject to small systematic effects that would lead to erroneous results. In order to investigate whether such a problem arises in this case, we use the GGA+U approach to Density Functional Theory to compare the results obtained by two methods of calculating these parameters: the dimer fragment approach and the periodic method, and relate them to both experiment and previous calculations performed using the hybrid approach.   

Time-Dependent Behavior in Arrays of Coupled Heisenberg Spin Chains

We employ matrix product state methods combined with data from exact solvability to study infinite arrays of coupled XXZ Heisenberg spin chains of finite length under a time dependent perturbation. We present results for both sudden changes (quantum quenches) as well more gradual changes in the interchain coupling. We benchmark our results and methods against perturbation theory as well as available equilibrium results on two dimensional Heisenberg models. We discuss these results in light of recent pump-probe resonant inelastic x-ray scattering experiments on the iridate compound Sr$_2$IrO$_4$.   

Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNb$_2$O$_6$ in a transverse field: Geometric frustration and quantum renormalization effects

We report extensive single-crystal inelastic neutron scattering measurements of the magnetic excitations in the quasi 1D Ising ferromagnet CoNb$_2$O$_6$ in the quantum paramagnetic phase to characterize the effects of the finite interchain couplings. In this phase, we observe that excitations have a sharp, resolution-limited line shape at low energies and over most of the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the full bandwidth along the strongly dispersive chain direction and resolve clear modulations of the dispersions in the plane normal to the chains, characteristic of frustrated interchain couplings in an antiferromagnetic isosceles triangular lattice. The dispersions can be well parametrized using a linear spin-wave model that includes interchain couplings and further neighbor exchanges. The observed dispersion bandwidth along the chain direction is smaller than that predicted by a linear spin-wave model using exchange values determined at zero field. We attribute this effect to quantum renormalization of the dispersion beyond the spin-wave approximation in fields slightly above the critical field, where quantum fluctuations are still significant.   

\textbf{Quantum Acoustic Magnetic Resonance Imaging and Spectroscopy:}

We present a new modality to characterize single molecule and molecular magnets and propose that it can be used as a powerful spectroscopy and imaging tool. Heisenberg type Hamiltonians representing realistic molecules with appropriate crystal field terms are solved and the magnetic field dependence of the resulting quantum spin energy levels enumerated. The results through thermodynamic identities yield the bulk modulus which is shown to be sensitive to the crystal field parameters at low temperatures. Thus high field low temperature measurements of the sound velocity in molecular and single molecule magnets open the road to a completely new method of understanding such systems.   

Molecular quantum magnetism with strong spin-orbit coupling in inorganic solid Ba$_{3}$Yb$_{2}$Zn$_{5}$O$_{11}$

The molecular magnet, assembly of finite number of spins which are isolated from environment, is a model system to study the quantum information process such as the qubit or spintronic devices. In past decades, the molecular magnet has been mostly realized in organic material, however, it has difficulty synthesizing materials or controlling their properties, meanwhile tremendous endeavors to search inorganic molecular magnet are continuing. Here, we propose Ba$_{3}$Yb$_{2}$Zn$_{5}$O$_{11}$ as a candidate of inorganic molecular magnet. This material consists of an alternating 3D-array of small and large tetrahedron containing antiferromagnetically coupled four pseudospin-1/2 Yb ions, and magnetic properties are described by an isolated tetrahedron without long-range magnetic ordering. Inelastic neutron scattering measurement with external magnetic field reveals that extraordinarily huge Dzyaloshinsky-Moriya (DM) interaction originating from strong spin-orbit coupling in Yb isospin is the key to explain energy level of tetrahedron in addition to Heisenberg exchange interaction and Zeeman effect. Magnetization measurement shows the Landau-Zener transition between avoided crossing levels caused by DM interaction.