Session A02: Condensed Matter I

Multimagnon dynamics and thermalization in the S=1 easy-axis ferromagnetic chain

While most isolated quantum systems prepared out of equilibrium eventually thermalize under time evolution, this is not guaranteed. Motivated by recent experiments in ultra cold-atomic settings [W.C. Chung et al., Phys. Rev. Lett. 126, 163203 (2021)], we investigate the dynamics and effective interactions of magnon quasiparticles in a ferromagnetic spin-1 Heisenberg chain with easy-axis onsite anisotropy. We show how the binding energy of multiple interacting magnons decreases due to effective mutual attraction, and how this energy scale is relevant for dynamics. Working with a proposed modification of the cold atoms experiment, where the magnon density can be tuned, we simulate these spin chains using the matrix product state based time-evolution block decimation algorithm. We show that the system can exhibit either (rapid) thermalization or many-body coherent oscillations depending on the magnon density and the strength of the easy-axis anisotropy (which controls the effective attraction between the magnons). The dynamical oscillations, akin to those reported for many-body quantum scars, are explained with an analytic approximation that is accurate in the limit of small anisotropy. Finally, we discuss how our predictions can be tested in future experimental settings.   

Generating short wavelength spin waves in Y-shaped Permalloy structures

Short wavelength or high wavevector spin waves are desirable for magnonic applications as devices scale to the order of the spin wave wavelength (micrometers to nanometers). Methods of generating high wavevector spin waves involving tapered waveguides, magnetic gratings, and nonlinear effects have been proposed in previous works but have their limitations. In this work, high wavevector spin waves are shown to be efficiently generated in Y-shaped microstructures. The structures are made of 2.7-$\mu $m wide and 40-nm thick Permalloy branches. Spin waves were driven in the arms of the structure with a microstrip antenna and channeled into the base. An in-plane external magnetic field was applied perpendicular to the base, and at an angle with the arms. Experiments conducted with micro-focus Brillouin light scattering (BLS) and calculations conducted with micromagnetic simulations reveal that spin waves excited in the arms undergo a process of wavevector up-conversion as they transition into the base. BLS measurements show additional spin wave frequencies in the base of the structure as compared to a straight microstrip. When compared to dispersion relations, the additional frequencies observed in the base correspond to those in the arms, which are at higher wavevectors in the base. These results represent new opportunities for efficiently generating short wavelength spin waves, which may be useful in nanomagnonic applications.   

Optimizing Fermion Nodes With Diffusion Monte Carlo and Gradient Descent

We present a method for optimizing the fermion ground state nodes using only diffusion Monte Carlo (DMC) with gradient descent. The method iteratively shifts the parameters of an arbitrary node-fixing trial function in the opposite direction of the DMC energy gradient. The energy gradient is calculated from DMC walker distributions by one of three methods we derive from an analytical expression. We combine our gradient calculation methods with various gradient descent algorithms and apply these "training" algorithms to trial functions of Be, Li$_2$, and Ne, all consisting of a single Slater determinant with randomized parameters. This dramatically improves the nodes to the same level as those optimized by variational Monte Carlo. Our method therefore enables DMC to be independent, departing from the standard procedure of optimizing the nodes with a non-DMC scheme such as variational Monte Carlo, Density function theory, or configuration interaction based calculation, which do not directly minimize the DMC energy.   

An Experimental Analysis of [(CH$_{3})_{3}$NH]CoCl$_{3}$ as a One-Dimensional Ising System Model

The room temperature single crystal structure of linear magnetic chain compound [(CH$_{3})_{3}$NH]CoCl$_{3}$ is reported, along with low temperature magnetic susceptibility and heat capacity measurements. [(CH$_{3})_{3}$NH]CoCl$_{3}$ single crystals were grown through slow evaporation and seed crystal methods. The compound crystallizes an orthorhombic structure with a$=$ 7.2716 {\AA} b$=$ 8.0983 {\AA} and c$=$ 16.6473 {\AA}, in accordance with previous literature$^{1}$. Small crystals grown initially via aqueous solvent evaporation were used to seed a supersaturated aqueous solution, resulting in crystals larger than previously reported. These crystals also experienced sensitivity to the Florida ambient, with crystals acquiring enough water from the humid air to dissolve. [(CH$_{3})_{3}$NH]CoCl$_{3}$ is of interest because it structurally realizes a one-dimensional spin Ising system. The Ising model is investigated by heat capacity and magnetic susceptibility measurements, the latter by applying magnetic fields perpendicular to the c- and b-axis at low temperature, using a SQUID magnetometer.   

Synthesis, magnetic and transport properties of new ternary silicide EuPd$_{3}$Si$_{2}$.

More than a hundred ternary borides, gallides and silicides are known which crystallize with a great variety of structure types which can be derived from the hexagonal CaCu$_{5}$ type (hexagonal, \textit{P6/mmm). }However, till today, none of the ternary silicide is reported being formed using Pd and Eu. We have recently synthesized single crystals and powder sample of the new phase EuPd$_{3}$Si$_{2}$. Single crystal data confirms that EuPd$_{3}$Si$_{2}$ crystallizes in orthorhombic symmetry with space group \textit{Imma }(pseudohexagonal) at room temperature. The lattice parameters are a $=$ 7.1463(3), b $=$ 10.0711(4), c $=$ 5.7469 (2) {\AA}. The energy dispersive X-ray spectroscopy measurements on the polycrystalline pellet further confirm the stoichiometry to be EuPd$_{3}$Si$_{2}$. Bulk magnetization and specific heat measurements have been performed on single crystals, indicating ferromagnetic order at a temperature T$_{C}$ of 78 K. A metamagnetic transition is observed near 5 K in both the magnetization and specific heat data, and resistance measurements on single crystal sample also exhibit a signature at T$_{C}$, consistent with magnetic long-range order.   

Multicritical Bifurcation and Weak First-order Phase Transition in a Three-dimensional, Three-state Ising Antiferromagnet

The Blume-Capel model generalizes the Ising model to three states, $\{-1,0,+1 \}$, with one interaction constant, $J$, and two fields: $H$ controlling the $+1/-1$ balance, and $D$ controlling the density of ``vacancies” ($0$). The antiferromagnetic (AFM) version ($J < 0$) possesses surfaces of second-order phase transitions between ordered AFM phases and a disordered phase at high temperature, and one of first-order transitions separating the ordered phases from a uniform phase of mostly 0 at large $D$. These surfaces join smoothly along a line of tricritical points. In 3D (but not in 2D), this line bifurcates into a line of critical end points and a surface of weak first-order transitions [J.D. Kimel and Y.L. Wang, J. Appl. Phys. 69, 6176 (1991)]. We consider the bifurcation region for 3D in detail by standard Monte Carlo simulations of lattices up to $32^3$ sites. Phase transitions were identified using finite-size scaling of order-parameter histograms, susceptibilities, and fourth-order cumulants. We identify the two phases separated by the first-order surface as two AFM ordered phases, one with a low vacancy density at temperatures below the transition, and one with a higher vacancy density above the transition. The density changes abruptly across the transition.