Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session V40: Phase Transition and Spin GlassFocus Live
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Sponsoring Units: GMAG DMP Chair: Christina Psaroudaki, Caltech |
Thursday, March 18, 2021 3:00PM - 3:12PM Live |
V40.00001: Complex anisotropic magnetic phase diagram of EuGa2Al2 Kevin Allen, Jaime Moya, Shiming Lei, Emilia Morosan In many rare-earth based intermetallic compounds, the interplay between structural, magnetic and electronic properties gives rise to emergent, competing phenomena like unconventional superconductivity, heavy fermion behavior intermediate valence, and quantum criticality resulting in complex phase diagrams. Often the competing ground states can be tuned by magnetic field, pressure or chemical substitution. In this contributing, we study such emergent competition in EuGa2Al2 as a function of magnetic field and temperature. EuGa2Al2 is an isostructural compound to the tetragonal parent compounds EuX4 (X = Ga, Al) where X is separated into two sublattices which can be preferentially occupied [1]. Eu exists in its magnetic 2+ state in EuGa2Al2 which orders antiferromagnetically at TN ~ 20K, with multiple metamagnetic transitions below TN and suspected charge order slightly above TN [1]. We map out the high-resolution field – temperature phase diagram revealing a complex anisotropic behavior with many competing magnetic phases. Together with magneto-electrical transport, thermodynamic, and x-ray diffraction measurements, we discuss the correlations between its structural, magnetic and electronic properties that give rise to the complex phase diagram. |
Thursday, March 18, 2021 3:12PM - 3:24PM Live |
V40.00002: Finite-temperature dynamics and the role of disorder in nearly-critical Ni(Cl1-xBrx)24SC(NH2)2 Leonardo Facheris, Dominic Blosser, Kirill Povarov, Robert Bewley, Severian Gvasaliya, Andrey Zheludev Time-of-flight neutron spectroscopy is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet Ni(Cl1-xBrx)24SC(NH2)2, x = 0.14(1), tuned close to the chemical-composition-induced soft-mode transition. The local dynamic structure factor shows ω/T scaling behavior characteristic of a quantum critical point. The deviation of the measured critical exponent from spin-wave theoretical expectations is attributed to disorder, which is also responsible for the localization of excitations above the magnon band. A simple model based on strong-bond dimers associated with Br-impurity sites can account for their energy, structure factor, and measured intensity [1]. |
Thursday, March 18, 2021 3:24PM - 3:36PM Live |
V40.00003: Thermal Response Functions of Insulating Magnets Near and Beyond Critical Temperature Caitlin Carnahan, Yinhan Zhang, Di Xiao Thermal response coefficients of magnetic materials are of significant interest in the field of spintronics; understanding spin dynamics in response to a thermal gradient paves the way for energy-efficient spin manipulation via thermal fluctuations. We investigate thermal response in magnetic systems, particularly near and above the critical temperature, by simulating the dynamics of these systems and calculating the spin and energy currents that appear. Applying linear response theory, we predict the thermal conductivities of both spin and energy, which provide insight into how thermally-induced fluctuations impact transport in magnetic systems and deepen the understanding of the general effect of fluctuations on non-trivial magnetic topology. |
Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V40.00004: Magnetic transition driven by chemical substitution in Cs1−xRbxFeCl3 Lena Stoppel, Shohei Hayashida, Zewu Yan, Severian Gvasaliya, Andrey Podlesnyak, Andrey Zheludev We report the observation a chemical-substitution driven phase transition from a gapped quantum paramagnetic phase to one with long range order in Cs1-xRbxFeCl3. The x = 0 compound in this series of triangular-lattice antiferromagnets has a spin-singlet ground state due to strong easy-plane magnetic anisotropy. In contrast, the x = 1 material orders magnetically in a 120° structure [1]. Calorimetric and magnetic experiments performed on a series of samples with 0 ≤ x ≤ 1 reveals that in the low-temperature limit magnetic order appears at x ~ 0.35. Inelastic neutron scattering experiments show that this coincides with the closure of the gap in the spin excitation spectrum. It appears that disorder effects in this material are more pronounced than those in the only other known phase transition of this type, namely in DTNX [2]. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V40.00005: Variational wave functions for spin-phonon models Francesco Ferrari, Federico Becca, Roser Valenti The existence and stability of spin-liquid phases represents one of the central topics in the field of frustrated magnetism. In the last few years, a large theoretical effort has been devoted to proposing and studying frustrated spin models which could host spin-liquid ground states. Although several examples of well-established spin-liquid phases are now available, the question of the stability of these states to the coupling between spins and lattice distortions (i.e. phonons) has been scarcely investigated. As suggested by the well-known one-dimensional case, the effect of phonons can cause a Peierls instability of spin-liquid phases towards the formation of valence-bond order. |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V40.00006: Origin of Memory in Spin Glass Dynamics Raymond Orbach The memory effect is one of the most mysterious dynamical processes in spin glasses. It evolves from waiting at a temperature T1 < Tg for a time tw, where Tg is the spin glass transition temperature, then reducing the temperature further to T2 . The spin glass magnetization behaves as though there has been no waiting at T1 during the temperature drop, and is referred to as "rejuvenation" or "temperature chaos." Yet, when the spin glass is warmed back to T1 the previous waiting time effect is recovered. This is termed "memory." Equilibrium simulations by Fernandez et al. [Europhys. Lett. 103, 67003 (2013)] have shown that chaos is driven by rare events. We argue that the length scale for chaos takes place within the spin glass correlation length ξ(tw ,T). This would be true both at equilibirium and in the non-equilibrium conditions of experiment. But if temperature chao is rare, some correlated regions of the spin glass will not experience chaos. Hence, their properties will be reversible, and are responsible for memory. Experimental evidence for this interpretation will be given. |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V40.00007: Probing spin-glass dynamics with 1/f Noise David Harrison, E. Dan Dahlberg, Raymond Orbach We have measured the 1/f noise in the electrical resistance of CuMn wires of varying thickness (10nm-80nm) in order to explore the dynamics of the spin-glass state. We use a cooling protocol suitable for comparison between experiment and recent simulations. We use the fluctuation dissipation theorem to compare our measurements with earlier measurements made of the out-of-phase component of the ac susceptibility in thin films and find good agreement. The frequency dependence of the noise indicates a crossover from two-dimensional to three-dimensional dynamics with increasing thickness. We provide a physical interpretation of a previously phenomenological result for this frequency dependence. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V40.00008: Field-induced quantum critical point in itinerant antiferromagnet Ti3Cu4 Jaime Moya, Alannah Hallas, Chien-Lung Huang, Vaideesh Loganathan, Lazar Kish, Adam Aczel, James W Beare, Yipeng Cai, Graeme Luke, Franziska Weickert, Andriy Nevidomskyy, Christos Malliakas, Mercouri Kanatzidis, Emilia Morosan, Kyle Bayliff Magnetism exists on a spectrum spanning the local to itinerant limit. Theory in the local moment extreme is experimentally confirmed in magnetic insulators where the magnetism originates from unfilled electronic shells. However, in the itinerant limit, where magnetism stems from band effects, experiment lags theory as there are only three known purely itinerant magnets Sc3In, ZrZn2, and TiAu, raising the need for discovery of new such systems. Also, it is of interest to understand what happens as magnetic order is suppressed to T=0 via various tuning parameters towards a quantum critical point (QCP). |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V40.00009: Measuring hybridized electro-nuclear modes at a quantum phase transition Matthew Libersky, Ryan D McKenzie, Daniel Marc Silevitch, Philip Stamp, Thomas F Rosenbaum Understanding the excitations, fluctuations, and dynamics at a quantum phase transition is an important research thrust in pure condensed matter and atomic physics, as well as applications such as adiabatic quantum annealing engines. One of the key open questions in this area is whether critical excitations remain when coupled to an external spin bath, as is present for many real-world implementations of quantum systems. Here, we directly measure the low energy excitation modes of a well-known realization of the quantum Ising model in transverse field, LiHoF4, using microwave spectroscopy techniques to probe energies below what is accessible via neutron scattering. We also measure the effect of a longitudinal magnetic field on the gap of the low energy collective mode. These results suggest that quantum criticality persists in the presence of a spin bath and that similar modes may exist in other quantum Ising systems, including adiabatic quantum computers. |
Thursday, March 18, 2021 4:48PM - 5:00PM Live |
V40.00010: Phases of quantum dimer magnets with strong spin-orbit coupling Michael Flynn, Thomas Baker, Rajiv Ranjan Singh Quantum dimer magnets are among the simplest examples of a quantum magnet, which are phases that cannot be understood without including the effects of entanglement. Typically, dimer magnets are characterized by spin singlet ground states that exhibit field-driven phase transitions in which bosonic (triplon) excitations are forced to condense. These phase transitions are naturally understood in the language of Bose-Einstein condensation (BEC), and give rise to symmetric dome-like structures separating phases in the magnetic field - temperature plane. Recent data on the strongly spin-orbit coupled dimer magnet Ytterbium Silicate has revealed an asymmetric BEC dome, with an additional unexpected phase transition within the dome. We present a model which reproduces the essential elements of this experimental data, and shed light on the nature of the universal physics associated with the unexpected phase transition. We argue that a weak breaking of crystallographic (inversion) symmetry is crucial to the observed physics. Via mappings onto hardcore bosons, we also comment more generally on the nature of phase transitions of dimer magnets with anisotropic interactions. |
Thursday, March 18, 2021 5:00PM - 5:12PM Live |
V40.00011: Experimental investigation into doped Fe2P compounds for magnetocaloric refrigeration Ethan Gibson, Benjamin Frandsen Magnetocaloric refrigeration holds promise as an efficient, low-waste method of refrigeration. Magnetocaloric compounds exhibit a reversible temperature change with application and removal of an external magnetic field due to the exchange of entropy spin and lattice systems across a magnetic phase transition. We performed muon spin relaxation experiments on compounds of Fe2P substituted with Mn and Si, which are promising candidates for practical applications. The results provide detailed information about the characteristics of the magnetic phase transitions in these compounds. We compare and contrast the observed behavior in the pure and doped compounds and offer insights into how this information can guide future applications of these materials. |
Thursday, March 18, 2021 5:12PM - 5:24PM Live |
V40.00012: Critical properties of randomly mixed magnetic model of Ising, six-state clock, and XY spins on the square lattice Tasrief Surungan, Aron Wiliyam Santo Mina, Zohar Nussinov Randomness is an important factor for phase transition and may influence phase transitions in a variety of ways. In the present study, we probe a new type of randomness:- that of a fluctuating number of components and/or allowed values at different lattice sites. In particular, we investigate a random mixture of Ising, six-state clock, and XY spins on a square lattice. As known, the critical behavior of each of the corresponding pure realizations of these models is governed by its own fixed point. We perform Monte Carlo simulation using a Wang-Landau algorithm to elucidate the behaviour of this new mixed model in which only the limiting cases correspond to known universality classes |
Thursday, March 18, 2021 5:24PM - 5:36PM Live |
V40.00013: Temperature Chaos in a Spin Glass Observed using the Correlation Length as a Caliper Qiang Zhai, Raymond Orbach, Deborah Schlagel We have developed a protocol to observe temperature chaos in a CuMn spin glass. Specifically, temperature cycling curves were compared with a reference curve, the latter without a temperature drop. The temperature cycling curves are shifted in time to overlap the reference curve at long times for a given temperature drop. Departures from the reference curve set in at a critical temperature change, indicating temperature chaos. For larger temperature drops, the temperature cycling curves depart even further from the reference curse, suggesting enhanced chaos. The results support the Bray-Moore length scale argument that temperature chaos sets in when the chaos length equals the spin glass correlation length. To extract the chaos exponent, the spin glass correlation is used as a “caliper” for the chaos length. The extracted chaos exponent is found near unity using different approximation schemes, agreeing for the first time with previous theoretical analysis and numerical simulations. Our results also demonstrate that temperature chaos occurs under non-equilibrium conditions. |
Thursday, March 18, 2021 5:36PM - 5:48PM Live |
V40.00014: Control of direct exchange and superexchange interactions with DC electric fields Shunsuke Furuya, Kazuaki Takasan, Masahiro Sato For many years, much experimental effort has been made for the challenging task of searching for an experimental realization of a theoretical model that can host a novel quantum phase of matter. In general, it is even more challenging to obtain such a model compound with an appropriate parameter set to realize the phase of interest. Accordingly, it is important to control and tune a microscopic Hamiltonian of the model compound by an external field. Recently, two of the authors, Takasan and Sato, discussed controls of phases of quantum spin systems by applying a DC electric field to them. We pointed out that the DC electric field controls the spatial anisotropy of the quantum spin system. In this presentation, as a natural and important extension of our previous work, we discuss DC-electric-field controls of superexchange interactions of geometrically frustrated quantum spin systems that will allow for tuning the microscopic Hamiltonian without changing the composition of the compound. In particular, we give our attention to a way to tune interaction parameters without affecting the spatial anisotropy. |
Thursday, March 18, 2021 5:48PM - 6:00PM On Demand |
V40.00015: Finite-size Scaling of O(n) Systems at the Upper Critical Dimensionality Jian-Ping Lv, Wanwan Xu, Yanan Sun, Kun Chen, Youjin Deng We address the logarithmic finite-size scaling (FSS) of the O(n) symmetry at the upper critical dimensionality. We establish an explicit scaling form for the free energy density, which simultaneously consists of a scaling term for the Gaussian fixed point and another term with multiplicative logarithmic corrections. In particular, we conjecture that the finite-size critical two-point correlation exhibits a two-length behavior, which is governed by Gaussian fixed point at shorter distance, and enters a plateau at larger distance whose height decreases with system size in a power law corrected by a logarithmic exponent. We carry out extensive Monte Carlo simulations for n-vector models, and obtain solid evidence supporting the conjectured scaling forms from the FSS of macroscopic quantities as well as the two-point correlation. According to the quantum-to-classical mapping, the three-dimensional quantum O(n) systems are at the upper critical dimensionality. Hence, the present study is of practical relevance to a large number of experimental systems including quantum magnetic materials, Josephson junction arrays, and ultracold atomic systems. |
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