Session Z53: Novel Magnetism in Low Dimensional Systems

From the transverse-field Ising chain to the quantum E8 integrable model: Theoretical progress and experimental realization

Exotic thermodynamics and excitations can emerge in the vicinity of a quantum phase transition. In the talk, I will first detailedly discuss the very unique quantum criticality for the Grüneisen ratio in the transverse field Ising chain (TFIC) [1], which then serves as a smoking gun to identify the underlying TFIC universality observed in quasi-one-dimensional (1D) antiferromagnetic material BaCo₂V₂O₈ with transverse field applied along [110] direction [2]. From systematic quantum critical analysis for the effective model of the material [3], we confirm SrCo₂V₂O₈ with field applied along [100] direction can also accommodate the TFIC universality with much weaker magnetic field [4]. Furthermore, when the quantum critical point (QCP) of the TFIC is perturbed by a longitudinal magnetic field, it was predicted that its massive excitations are precisely described by the exceptional E₈ Lie algebra. Here we show an unambiguous experimental realization of the massive E₈ phase in the material BaCo₂V₂O₈, via NMR and inelastic neutron scattering measurements, and detailed theoretical analysis [5, 6, 7]. The large separation between the masked 1D and 3D QCPs of the system allows us to identify, for the first time, the full 8 single-particle E₈ excitations and various multi-E₈-particle states in the spin excitation spectrum. Our results open new experimental and theoretical route for exploring the dynamics of quantum integrable systems and physics beyond integrability, and thus bridge key physics in condensed matter and statistical field theory.

[1] Phys. Rev B 97, 245127 (2018).

[2] Phys. Rev. Lett. 120, 207205 (2018).

[3] J. Phys. Condens. Matter. 32, 045602 (2020).

[4] Phys. Rev. Lett. 123, 067203 (2019).

[5] Phys. Rev. B 101, 220411(R) (2020).

[6] Phys. Rev. B 103, 235117 (2021).

[7] Phys. Rev. Lett. 127, 077201 (2021).   

Density functional theory calculations of single-molecule magnets Co3(SALPN)2(O2CCH3)2⋅R2

In search of candidate magnetic molecular systems that emulate Majorana zero modes, we perform density functional theory (DFT) calculations for Co trimers Co₃(SALPN)₂(O₂CCH₃)₂⋅R₂, where R is an OCH₂, OCHNH₂, or OCHN(CH₃)₂ solvent molecule. The three Co atoms form a one-dimensional chain, and each Co atom exhibits axial spin with S=3/2. We extract the exchange coupling constant and the local axial and rhombic zero-field splitting (ZFS) parameters based on DFT total energies. According to our calculations, the ratio between the exchange coupling constant and the local axial ZFS parameter is 7.3 for the end Co atoms and 5.7 for the center Co atom; the local magnetic easy axis of the center Co atom differs from that of an end Co atom by 42.9 degrees. These results provide a starting point for mapping the spin-3/2 Hamiltonian to an effective spin-1/2 Hamiltonian, which indicates the existence of Majorana zero modes. We will also analyze the d-orbital occupation matrix and discuss the necessity to control it for obtaining reliable results.    

Ultra-low Thickness Titanium Nitride Thin Films for Spintronic Devices

TiN thin films were deposited using a pulsed laser deposition method in the thickness range of 9-45 nm. Temperature-dependent resistivity measurement showed a metal-to-insulator (MI) the transition of the TiN thin films below 15 nm. This value represents a critical thickness at which the properties of TiN films are strikingly different from the bulk TiN material. The thickness-dependent MI transition was free from structural phase transition and can be attributed to the localization of 3d¹ electrons in the valence band. This localization takes place due to the separation in the energy states larger than the gaps between the valence and conduction bands. The analysis of transport resistivity data demonstrated that the Arrhenius law governs the transport properties of TiN films in the 300-350 K range, while the transport properties are governed by a thickness-dependent variable range hopping mechanism below 300 K. Both metallic and semiconducting TiN films showed ferromagnetic behavior at room temperature. There was a marked correlation between coercivity (H_c) and saturation magnetization (M_s) with TiN film thickness. At room temperature, H_c increased from 66 to 134 Oe as the thickness decreased from 45 nm to 9 nm. The M_s at room temperature was also highest for the 9 nm sample (2.8 emu/g). The observed magnetic behavior of TiN films due to the localized 3d¹ electron. These electrons become more localized with the decrease in film thickness due to the size-dependent enhanced separation of energy states.    

Avoided Quantum Critical Point in La5(Co1-xNix)2Ge3

La₅Co₂Ge₃ is an itinerant ferromagnetic system that undergoes a second order phase transition at 3.8K with a low-field saturated moment of about 0.1 μ_B/Co¹, making it a promising candidate for tuning the transition to even lower temperatures with a non-thermal tuning parameter like pressure, chemical substitution, or magnetic field. Under applied pressure, the FM transition in La₅Co₂Ge₃ is weakly suppressed to 3 K at 1.7 GPa, upon which a new possibly magnetic, phase emerges upto 5.12GPa². The new phase is presumed to be antiferromagnetic, meaning the FM QCP appears to be avoided in favor of an AFM ground state. Here we further study the La₅Co₂Ge₃ system using Ni substitution. We synthesize single crystals of La₅(Co_1-xNi_x)₂Ge₃ with x = 0.01–0.10 and construct a T-x phase diagram from magnetization, transport, and thermodynamic measurements. Details of this diagram will be discussed and compared to the earlier T-P work.

1.           Saunders, S. M. et al., Phys Rev B 101, (2020).

2.           Xiang, L. et al., Phys Rev B 103, (2021).   

The temperature-pressure phase diagram of La5(Co1-xNix)2Ge3; x = 0.03

La₅Co₂Ge₃ is an itinerant ferromagnet (FM) system that undergoes a second order phase transition at 3.8 K with a low-field saturated moment of about 0.1 μ_B/Co¹, suggesting that the magnetic order is could be fragile. Under applied hydrostatic pressure, the FM quantum critical point in undoped La₅Co₂Ge₃ is avoided by the emergence of a different, likely antiferromagnetically ordered, state²­. Our recent studies on La₅(Co_1-xNi_x)₂Ge₃ with x = 0.01 - 0.10 show that the system can be tuned from FM ground state to a different, antiferromagnetic, ground state with a clear and well-defined signature in magnetization as well as resistivity. We now investigate the high pressure electrical properties of La₅(Co_1-xNi_x)₂Ge₃ with x = 0.03, where the sample has just crossed into the clean AFM ground state, to study the evolution of the AFM ordering in this system. The temperature-pressure magnetic phase diagram of La₅(Co_1-xNi_x)₂Ge₃; x = 0.03 is constructed. Details of this diagram will be discussed and compared to the T-P work in the undoped system.

 

1. Saunders, S. M. et al., Phys Rev B 101, (2020).

2. Xiang, L. et al., Phys Rev B 103, (2021).   

Probing Intermolecular Magnetic Interactions in a Metal Complex by Inelastic Neutron Scattering

Molecular magnetic materials, such as single-molecule magnets (SMMs) composed of a single paramagnetic metal center complexed by organic ligands, are of intense interest for their potential applications as traditional bits in high-density data storage devices or as qubits in quantum computers. It is generally assumed that these paramagnetic centers behave independently of one another. However, spin-spin couplings among the molecules are known to contribute to magnetic relaxation. Few techniques exist to directly probe the intermolecular magnetic interactions. We have utilized inelastic neutron scattering (INS) to reveal these interactions in a field-induced single-molecule magnet, Mn(TPP)Cl (TPP^2- = meso-tetraphenylporphyrinate) as an oscillatory Q-dependence of the magnetic transition. We analyze the INS results using a combination of Heisenberg spin-spin exchange for intermolecular spin interactions in combination with single-ion excitations.    

Machine Learning Study of the Magnetic Ordering in Two-dimensional Materials

The advent of two-dimensional (2D) materials opened new arenas for magnetic compounds, even when classical theories discourage their examination. The recent experimental discovery of 2D magnetic materials has inspired fundamental questions about the physical mechanism behind the magnetic ordering. Despite the huge interest raised by 2D magnetic materials, there are no a priori rules or trends allowing the prediction and understanding of this class of functional compounds. Here, we perform a machine learning study to predict and understand the tendency in the space of atomic species and 2D structures for a material to be classified as i) magnetic or non-magnetic using a random forest algorithm coupled to the SHAP method for model interpretability; and ii) ferromagnetic or antiferromagnetic based on the SISSO method. The ML models (i.e., a materials map that is function of the composition, atomic properties, and crystal symmetry) provide an accuracy higher than 85%. We find classification rules discriminating the existence of magnetism as well as the magnetic ordering type in terms of the strength of the spin-orbit coupling, the type of transition metal, the indirect bonding among transitions metals, and atomic properties of the constituent atoms, indicating new routes for experimental exploration.   

Magnetic and transport properties of single-crystalline EuPd3Si2

More than a hundred ternary borides, gallides and silicides are known which crystallize with a great variety of structure types that can be derived from the hexagonal CaCu₅ type (hexagonal, P6/mmm). However, no ternary silicide is reported being formed with Pd and Eu. We have synthesized single crystal and powder samples of the new phase EuPd₃Si₂ and investigated the crystallographic structure, magnetization, specific heat, and resistivity as a function of temperature and magnetic field. Single crystal data confirms that EuPd₃Si₂ crystallizes in the orthorhombic space group Imma (pseudohexagonal) with lattice parameters a = 7.1463(3), b = 10.0711(4), c = 5.7469 (2) Å. Long-range ferromagnetic order at T_C = 78 K and a possible spin rearrangement at T* = 5 K appear as distinctive features in all physical properties investigated. The electronic band structure calculations using the PBE+U method within the density functional theory approach corroborate the orthorhombic structure and ferromagnetic ordering of Eu²⁺ spins. The band structure calculations also show that the Pd derived d-band polarization is along the Eu f polarization, resulting in an enhancement of the spin moment in ordered states.   

Spin-Mediated Response in 2D Honeycomb Magnets

Two-dimensional honeycomb ferromagnets that include a second nearest neighbor Dzyaloshinskii-Moriya interaction reproduce the Haldane model and are therefore expected to exhibit a non-trivial magnon band topology. It has been shown that such a system gives rise to a thermal Hall effect via magnon carriers. Likewise, chiral honeycomb antiferromagnets may generate a spin Nernst effect in response to the application of a temperature gradient. Recent theoretical studies modeling these spin-mediated responses in honeycomb magnets have been restricted to the linear spin wave limit – and are therefore only valid at low temperatures – or otherwise neglect to incorporate higher-order spin fluctuations which are crucial in the vicinity of the critical temperature.  In this talk, we will describe our approach to model the thermal Hall effect and spin Nernst effect in honeycomb magnets, particularly near and beyond the critical temperature. We will also show that spin-mediated response bolstered by increasing thermal fluctuations can be particularly robust at high temperatures, a result that is unexpected in the traditional magnon picture.   

Spin dynamics of Dy2 molecules deposited onto micro-SQUID sensors

We report the results of ac susceptibility measurements performed down to very low temperatures (T > 13 mK) on thin layers of asymmetric Dy₂ molecular coordination complexes that have been proposed as candidates for the realization of 2-qubit quantum gates. The molecules are integrated into a μ-SQUID susceptometer by means of Dip Pen Nanolithography. Frequency-dependent susceptibility data measured on 5 and 20 molecular layers thick films are compared with similar results obtained for bulk polycrystalline samples. The results show that the magnetic anisotropy, exchange interactions and spin tunneling rates of Dy₂ units largely remain intact at the surface. Low-nuclearity lanthanide magnetic clusters might then provide suitable building blocks for the development of a scalable hybrid quantum architecture.   

Magnetoelastic coupling in α-RuCl3 and possible technological applications of α-RuCl3/graphene heterostructures

In this talk, I will focus on the first-principles results on the magnetoelastic coupling in α−RuCl₃ and the dependence of the magnetic coupling constants on strain effects. Different magnetic interactions are found to respond unequally to the variations in the lattice, with the Kitaev interaction being the most sensitive. We will discuss how uniaxial strain perpendicular to the honeycomb planes reorganizes the relative coupling strengths, strongly enhancing the Kitaev interaction while simultaneously weakening the other anisotropic exchanges under compression. Uniaxial strain may therefore pose a fruitful route to experimentally tune α−RuCl₃ nearer to Kitaev limit. I will also discuss the corresponding scenario when the strain changes from uniaxial to homogeneous. 

In heterostructure geometry with graphene, α−RuCl₃ shows interesting properties due to the proximity effect.  I will show how the charge transfer between α−RuCl₃ and graphene can be used for device applications in technology.